Tenders

Precision Radiating Optimum Power High Emitting Transmitter (PROPHET)

NAVAL RESEARCH LABORATORY

THIS IS A SOURCES SOUGHT NOTICE (SSN)/REQUEST FOR INFORMATION (RFI) AND NOT A SOLICITATION OR AN INDICATION THAT A CONTRACTUAL COMMITMENT WILL EXIST AS A RESULT OF THIS SSN. This announcement is not a formal Request for Proposals (RFP). This SSN/RFI is for informational and planning purposes only and does not guarantee that an award will be made. PROPOSALS ARE NOT REQUESTED AND ANY PROPOSASLS SBUMITTED IN RESPONSE TO THIS POSTING WILL BE DISCARDED AND NOT REVIEWED. The Government assumes NO financial responsibility for any costs incurred. A solicitation package is not available at this time. However, should such a requirement materialize, no basis for claims against NRL shall arise as a result of a response to this SSN/RFI or NRL’s use of such information either as part of our evaluation process or in developing specifications or work statements for any subsequent requirement. LOCATION : Contracting Office Address: NRL, 4555 Overlook Avenue, SW, Washington, DC 20375 OBJECTIVE: The requirements of this Statement of Work (SOW) include the development of hardware and software that support signal research for NRL Code 8140 used to support an overall space research program. In order to perform under this SOW, the contractor shall own, have lease to, or right to use the proposed antenna. The government shall inspect/assess the antenna before award to insure that the antenna is operational and accommodates the requirements in this SOW as part of a pre-award survey. The contractor shall provide scientific, technical and engineering research and development using a 30M Antenna system located in a 80,467-meter radius from the DoD space object ellipse center point of 36.597564 N,-121.897631 W. The requirements defined in this SOW are anticipated to be performed at this antenna facility and other NRL facilities. More details are specified in the attached Statement of Work document. REQUIRED CONTRACTOR RESPONSE FORMAT: Interested parties, at a minimum, shall supply all of the be

Z--Nathaniel Washington Power Plant Turbine Repair: Construction Indefinite Delive

PACIFIC NW REGION PN 6615

Synopsis of Proposed Contract Action Description The Bureau of Reclamation, Columbia-Pacific Northwest Region, Columbia-Cascades Area Office has a need for a five (5) year Indefinite-Delivery/Indefinite Quantity (IDIQ) contract type for construction services for Nathaniel `Nat¿ Washington Power Plant Turbine Repair, Columbia Basin Project, Washington. The work location will be at the Grand Coulee Dam and Power Plant, approximately 90 miles west of Spokane, Washington, and 60 miles north of Moses Lake, Washington, in Okanogan County, near the city of Coulee Dam. Principal components of work to be performed under these specifications include work necessary to access and repair turbine runners and stay vanes in the Nathaniel `Nat¿ Washington Power Plant, including construction of temporary scaffolding, inspection, weld preparation, welding, grinding, and coating. Estimated Principal Components of work to be performed includes the following: A. Inspecting turbine runner, discharge ring, and stay vanes. B. Installing and removing draft tube platforms. C. Building and removing temporary scaffolding for accessing repair areas. D. Metal removal. E. Welding. F. Grinding. G. Coating. The North American Industry Classification System (NAICS) code associated with this project is 238190 ¿ Other Foundation, Structure, and Building Exterior Contractors. The small business size standard applicable to this project is $19.0 million in gross annual receipts averaged over the previous three 1-year accounting periods. The projected IDIQ ceiling amount is $13,500,000.00, representing the maximum cumulative value of all task orders issued under this contract. An unrestricted, full and open competitive solicitation under NAICS 238190 shall be issued. Bids will be accepted from all responsive and responsible entities. The clause FAR 52.225-11 Buy American-Construction Materials under Trade Agreements will be included in any forthcoming solicitation, therefore one or more of the items under

36C26326Q0507, Z1DA -PN: 568A4-26-SL001, Fire System Maintenance (HS)

NETWORK CONTRACT OFFICE 23 (36C263)

See Amendment 0001 for Solicitation for PN: 568A4-26-SL001, Fire System Maintenance FY26 A

Cherokee National Forest Project TN ERFO FS CHRKE804 2020-2(4)

693C73 EASTERN FED LANDS DIVISION

Sealed bids from certified HUBZone Small Business Concerns only will be accepted for the Cherokee National Forest, Project TN ERFO FS CHRKE804 2020-2(4), Solicitation Number 693C73-26-B-000010 located in Polk County, Tennessee. This project consists of repairing storm and flood damage on Little Gassaway Road (FS 45) and Sina Branch Road (FS 67). Project work includes grading, roadway resurfacing, drain dip construction, replacement of drainage culverts, slope stabilization, AOP structure, concrete box culvert, ditch reconditioning, drainage culvert cleaning, and other miscellaneous work. The project is expected to fall within the price range of $5,000,000 to $10,000,000. Bid documents should be issued on or about April 13, 2026. Bid due date and location will be specified on the SF 1442, Block 13a of the Solicitation. Use Internet address https://www.sam.gov (click on Contract Opportunities, then type “693C73” in the Search Contract Opportunities box and click the search button and all available projects will be listed) to check for the availability of advertisement documents. All documents can be directly downloaded from this website. Solicitation documents will not be mailed. Register to receive email notifications to be automatically notified when a document is added or updated for a specific project. If firms do not register as an interested vendor at this website, there will be no Interested Vendors List. Please send all questions concerning construction projects to eflhd.contracts@dot.gov. Include the solicitation number, project name and number, requesting firm and address, a point of contact and telephone number. Requests for 'faxing' or overnight mailing will not be accepted. IMPORTANT SAM REGISTRATION AND PAYMENT REQUIREMENTS In accordance with Federal Acquisition Regulation (FAR) 52.204-7 (DEVIATION NOV 2025), prospective contractors shall have an active Federal Government contracts registration in the System for Award Management (SAM) when submitting an

Levee Repairs at Saline County Levee District #2, Ray-Carroll Bottoms Consolidated Levee District of Carroll County, and Howard County Levee District #4

W071 ENDIST KANSAS CITY

The solicitation is amended (Amendment 0002) to revise the specifications at 01 50 00, para. 2.1.1; 01 50 00, para. 3.5; 01 57 19, para. 3.5; 35 41 00 Para. 3.17.2, add drawing C-0 to 35 41 00 Attachment 1, and update the wage determination. The solicitation is amended to change the site visit, bidder inquiry, and bid due dates. Also, RFO Clause 52.222-90 is added. The new site visit date is 7 May. The Army Corps of Engineers, Kansas City District has need for construction services for levee repair at the following sites: Saline County Levee District #2, Ray Carroll Levee, and Howard County Levee District #4.The scope of work consists of furnishing all equipment, labor, operators, supervision/site superintendent, quality control, supplies, materials, and incidentals necessary to satisfactorily perform and provide construction to potentially include, but not limited to earthwork operations, bringing in fill and borrow material from a pre-determined site, haul road development, clearing of debris as necessary, erosion repair, rutting repair, levee surfacing, various stone surfacing/placement, installation of stone armored protection, installation and rehabilitation of drainage structures and associated apertures, and establishment of turf for repairs of compromised landside, riverside, and crown levee sections.An organized site visit has been scheduled for 30 April at 10am. The magnitude of this project is represented by the following estimated price range: Between $250,000 and $500,000.

Historic Architecture, Conservation, and Engineeri

NER SERVICES MABO (43000)

Historic Architecture, Conservation, and Engineering (HACE) FY26 Preventative Conservation Services

127EAW26Q0003 - Plumas NF Bear Boxes

USDA-FS, CSA SOUTHWEST 6

Acceptance is made of Platinum Contracting's proposal submitted on 4/8/2026, in the total amount of $292,975.90 to perform the Plumas NF Bear Boxes project located within the Plumas National Forest.

15B20926Q00000010 AWARD HOT WATER BOOSTER

FCI MCKEAN

Y1DA--637-22-700 | EHRM Infrastructure Upgrades | Ashville, NC |

PCAC HEALTH INFORMATION (36C776)

The contractor shall provide all tools, equipment, materials, labor, supervision, and management to complete the construction of infrastructure upgrades to the campus. Construction practices shall comply with all VA solicitation drawings and specifications, VA regulations, procedures, and conditions for work at the medical center and any applicable codes as per requirements of all governing agencies. Work includes, but is not limited to, electrical (electrical panel upgrade, power (normal, emergency)), Bonding, UPS, Building Management System interfaces, asses for new and upgrades to existing HVAC, reconfiguration, expansion and renovation of existing space (demo, new construction, finishes, cable), Renovation Datacenter work (HVAC, UPS, Power distribution, cable management, expansion of space, fire suppression, finishes), communication infrastructure (new Data outlets, patch panels, upgrade to Cat 6A cable) in buildings as necessary, physical security upgrades, assess and upgrade fiber infrastructure backbone campus wide within buildings and between buildings, provide diversified path for wide area network connection, hazardous material abatement. Project buildings include: Buildings 47, 62, 14, 15, 9, 7, 6, 5, 4, 3, 2, 73, 70, 11, 12 and site. Contractor shall complete the intent of project 637-22-700 in strict accordance with the drawings and specifications. All work not specifically identified by the drawings and/or specifications but necessary for a complete system shall be included as if specifically identified in the Contract Documents.

METC Instructor Training Support Services

DEFENSE HEALTH AGENCY

I. HOSPITAL CORPSMAN BASIC (HCB) COURSE. The HCB Course prepares enlisted sailors to function as entry-level medical technicians in fixed and non-fixed medical facilities, performing duties to provide quality emergency, nursing and primary care procedures. The graduates shall demonstrate: the ability to comprehend, evaluate and apply information relevant to the role of a Hospital Corpsman; technical proficiency in entry-level skills required to fulfill the role of a basic Hospital Corpsman; and personal and professional behavior consistent with the expectations of a Navy sailor. The HCB Course is an apprentice level curriculum that is grounded partly in the Emergency Medical Technician realm as well as generalized medicine and nursing skills. There is no civilian credentialing body or professional certification that encapsulates the scope and the role of a Navy Hospital Corpsman that can be used as a model for this course. The course provides requisite knowledge, as well as simulated and clinical training in various aspects of emergency, nursing, and primary care. Subjects include medical terminology, anatomy & physiology, Basic Life Support (BLS), Tactical, Combat, Casualty Care (TCCC) as well as various aspects of nursing and primary patient care. Outcome-based practice and safety techniques are emphasized throughout the course. The HCB Course provides enlisted sailors basic knowledge of emergency medicine and nursing care. Graduates are awarded the following Navy Enlisted Classification (NEC): Navy B-300-0010 Hospital Corpsman Basic (HM-0000) Navy B-300-0004 Hospital Corpsman Advanced Technical Field Course (HM-ATF-0000) This course consists of multiple courses delivered in a single phase which consists of knowledge and skills training. The contractor shall provide education and training through lecture, hands-on instruction and clinical practice, guided discussion, demonstration, e-learning materials, simulations, and skills laboratory practice. Practical and wr

Permanent Bakeout Heaters for ALS-U

BERKELEY NATL LAB - DOE CONTRACTOR

As of 5/22/26 this RFQ is hereby modified to incorporate newly attached Question&Answer document (QuestionAnswer, AA-2026-07 Rev. 1.pdf) All other terms and conditions of this RFQ remain the same. _________________________________________________________________________ As of 5/20/26 this RFQ is hereby modified to incorporate newly attached Question&Answer document (QuestionAnswer, AA-2026-07.pdf) All other terms and conditions of this RFQ remain the same. _________________________________________________________________________ Dear Prospective Offeror: The University of California, on behalf of the Lawrence Berkeley National Laboratory (LBNL), would like to invite you to participate in small business set aside , Request for Quote (RFQ) No. AA-202 6 -07 for Permanent Bakeout Heaters that will be utilized in the ALS-U. The RFQ and associated documents (listed below) are attached: 0. RFQ and submission instructions 1. Sample Subcontract 2. Statement of Work 3. Past Performance Reference Information Form (submit with Quote regardless of value) 4 . Rep Cert Form (required with submission if total Quote exceeds $150K) *With your Quote submission, you must certify in writing that you meet and agree to all of the Mandatory Requirements outlined within Doc 0. RFQ AA-2025-07 and will manufacture per SOW. Timeline of Events: RFQ Release - May 14, 2026 Quotes Due - May 29, 2026

Analytical Capability to Link Data for Domestic and International Companies

DEFENSE CI AND SECURITY AGENCY

SOURCES SOUGHT ANNOUNCEMENT The Defense Counterintelligence and Security Agency (DCSA) is seeking sources for a business analytical capability able to link data, news, market research, and analytics, to identify trends, connections, supply chains, ownership, leadership, interlocking corporate relationships, business operations, financial viability, industry position, merger and acquisition activity (M&A), and identify risk in the commercial and financial community for domestic and international companies. CONTRACTING OFFICE ADDRESS: Requiring Activity/Program Office : Defense Counterintelligence and Security Agency (DCSA) Entity Vetting Directorate (EV); and Counterintelligence Directorate (CI) 27130 Telegraph Road Quantico, VA 22314 Contracting Activity : DCSA-Procurement Solutions (PRS) 27130 Telegraph Road Quantico, VA 22314 INTRODUCTION: This is a SOURCES SOUGHT TECHNICAL DESCRIPTION to determine the availability and technical capability of small businesses (including the following subsets, Small Disadvantaged Businesses, Certified 8(a), Service-Disabled Veteran-Owned Small Businesses and Woman Owned Small Businesses) to provide the required products and/or services. DCSA’s entity Vetting Directorate (EV), and Counterintelligence (CI) Directorate, are seeking information for potential sources of a business analytical capability able to link data, news, market research and analytics, to identify trends, connections, supply chains, ownership, leadership, interlocking corporate relationships, business operations, financial viability, industry position, merger and acquisition activity (M&A), and identify risk in the commerce and financial community for domestic and international companies to support national security. DISCLAIMER: THIS SOURCES SOUGHT IS FOR INFORMATIONAL PURPOSES ONLY. THIS IS NOT A REQUEST FOR PROPOSAL. IT DOES NOT CONSTITUTE A SOLICITATION AND SHALL NOT BE CONSTRUED AS A COMMITMENT BY THE GOVERNMENT. RESPONSES IN ANY FORM ARE NOT OFFERS AND THE GOV

SOLE SOURCE – NuSiL RTV COATING

NSWC CRANE

NuSil Technology, LLC- P/N: R-3904 (Parts A and B)- U/M=EACH= 1 Gallon/ Kit- Quantity 38 The proposed contract action is for the supplies or services for which the Government intends to solicit and negotiate with one source NUSIL TECHNOLOGY, LLC; Cage Code: 5U206, 1050 Cindy Ln, Carpinteria, CA 93013. under Simplified Aquisition Procedures. This is being solicited on a sole source basis due to form fit function. All responsible sources may submit a capability statement, which shall be considered by the agency. However, a determination by the Government not to compete with this proposed contract based upon responses to this solicitation is solely within the discretion of the Government. Information received will normally be considered solely for the purpose of determining whether to conduct a competitive procurement. Contractors must be properly registered in the System for Award Management (SAM). Offerors may obtain information on SAM registration and annual confirmation requirements by calling 866-606-8220 or via the internet at https://sam.gov. This solicitation is being issued pursuant to Simplified Aquisition Procedures. Synopsis exception FAR 5.202(a)(13) applies and Solicitation Number N00164-26-Q-0161 is hereby issued as an attachment hereto. All changes to the requirement that occur prior to the closing date will be posted to Contract Opportunities on https://sam.gov/. It is the responsibility of interested vendors to monitor https://sam.gov/ for any amendments, which may be issued to this solicitation. For changes made after the closing date, only those offerors that provide a quote will be provided any changes/amendments and considered for future discussions and/or award. Offers shall be e-mailed to sandra.e.gunderman.civ@us.navy.mil. All required information must be received on or before 27 MAY 2026, 3PM EASTERN TIME.

Brand New Snowmobiles

USDA-FS, CSA EAST 6

Sources Sought Notice - Registered Dietitian Services

NAVAJO AREA INDIAN HEALTH SVC

Sources-Sought Notice The Indian Health Service, Chinle Service Unit is issuing this sources-sought synopsis as a means of conducting market research (FAR 10.001) in identifying prospective sources to fill a government requirement for Registered Dietitian services to the Clinical and Community Nutrition Department at Chinle Comprehensive Health Care Facility (CCHCF). Refer to the attached Performance Work Statement for more detail. The applicable North American Industry Classification System (NAICS) code assigned to this requirement is 561320 – Temporary Help Services with a small business size standard of $34.0m. THIS IS NOT A SOLICITATION . Information collected from this notice is purely for market research only and will be used to determine the contracting office’s acquisition strategy including competition, set asides, and locating Indian-owned economic enterprises. Submission of any information in response to this market survey is purely voluntary. The government assumes no financial responsibility for any costs incurred in response to this request for information. Period of Performance: One (1) one-year Base Period Plus One (1) one-year Option Period. In accordance with FAR 4.1102(a), offerors and quoters are required to be registered in System for Award Management at SAM.gov, at the time an offer or quotation is submitted in order to comply with the annual representations and certifications requirements. Submission Instructions : Interested parties who consider themselves qualified to perform the above-listed services are invited to submit a response to this notice by June 5, 2026, at 5:00PM MST to Teola Autaubo, Purchasing Agent at teola.autaubo@ihs.gov with the subject line “SSN - IHS1521758 - Registered Dietitian Services. Your submission must include the following information, at a minimum: 1. Company Profile that includes: UEI, Name and Address, Primary POC information, business size and type of ownership, socio-economic status/participation such as 8(a

Warehousing and Distribution Services at DLA Distribution Barstow, California (DDBC)

DLA DISTRIBUTION

Supplemental Notice 1 - Site Visit and Conference Instruction and DRAFT Solicitation for Warehousing and Distribution Services at DLA Distribution Barstow, California (DDBC) - SP3300-26-R-5003.

GT Wash Rack Upgrade & Maintenance

FA4803 20 CONS LGCA

THIS IS A SOURCES SOUGHT ONLY – The Government is requesting information to determine the availability of qualified sources capable of providing a Ground Transportation Wash Rack Upgrade and Monthly Maintenance Services in accordance with the attached Statement of Work (SOW). At this time, the Government is only requesting to know if your company can provide these services and equipment. IF YES, please provide a copy of your capability statement to: Lt. Michael Mitchell (Contract Specialist) at michael.mitchell.66@us.af.mil Ms. Shaunannette Watson (Contracting Officer) at shaunannette.watson@us.af.mil DO NOT SUBMIT ANY QUESTIONS OR AN OFFER AT THIS TIME.

RFQ Drum, Shipping - 8110-00-030-7780

DLA TROOP SUPPORT

Solicitation: SPE8E9-26-Q-0417 has been posted to DIBBS and is available to submit quotations. This procurement is using the procedures of 41 U.S.C. 1901, as implemented by Class Deviation 2026-O0028 – Revolutionary Federal Acquisition Regulation (FAR) Overhaul Part 12, Defense FAR Supplement (DFARS) Part 212 and Revolutionary FAR Overhaul Part 12.201-1. The Government intends to issue a single award for the procurement of the National Stock Number (NSN) 8110-00-030-7780 – Drum, Shipping and Storage. The resultant contract will be Indefinite Delivery, Indefinite Quantity (IDIQ), Fixed Price Contract with Economic Price Adjustment (EPA) for DLA requirements. This acquisition is set aside 100% for small business participation. The contract will have a base of one (1) year, with four (4) one-year options, for a total possible five (5) year contract. The Government will make an award as a result of this solicitation, to the responsible offeror, offering the lowest evaluated price, and taking no exception to the terms and conditions set forth in the solicitation, including offering an item that complies with the applicable item description. All proposals MUST be submitted via DIBBS or email and be received by the close of the solicitation – Tuesday, June 22, 2026, at 3:00 PM Eastern Standard Time. Proposals may be submitted via email to Acquisition Specialist, Quiana Bowser, Quiana.Bowser@dla.mil. Request for Quotation can be located on the DLA Internet Bid Board System (DIBBS): https://www.dibbs.bsm.dla.mil/RFQ/

SOLICITATION - National Capital Region (NCR) Multiple Award Task Order Contract (MATOC)

W6QM MICC TSC

W51EW7-26-R-A002 AMENDMENT 0005 1. Mission and Installation Contracting Command (MICC), Theater Support Center (TSC), hereby issues Amendment 0005 against Solicitation W51EW7-26-R-A002 for the National Capital Region (NCR) Multiple Award Task Order Contract (MATOC), in support of U.S. Army Installation Management Command (IMCOM) Installation Directorate - Sustainment (ID-Sustainment). 2. The purpose of Amendment 0005 is as follows: - Extend the Solicitation Closing Date and Time from 03JUN2026 to 17JUN2026 at 2:00 pm CT. - Notify all interested Offerors that a future amendment is forthcoming providing answers to previously submitted questions. - Offerors shall acknowledge this Amendment within their proposal submission. 3. All other terms and conditions of Solicitation W51EW7-26-R-A002 remain unchanged and in full effect. ************************************************************** W51EW7-26-R-A002 AMENDMENT 0004 1. Mission and Installation Contracting Command (MICC), Theater Support Center (TSC), hereby issues Amendment 0004 against Solicitation W51EW7-26-R-A002 for the National Capital Region (NCR) Multiple Award Task Order Contract (MATOC), in support of U.S. Army Installation Management Command (IMCOM) Installation Directorate - Sustainment (ID-Sustainment). 2. The purpose of Amendment 0004 is as follows: a) Revise NCR MATOC SOW dated 24APR2026; para 1, Contract Description; Incorporating On-Ramp/Off-Ramp Language b) Revise NCR MATOC Rate Sheet - NTE v2; to include Key Personnel Position "Site Safety & Health Officer" c) Revise Past Performance Questionnaire due date to 03 June, 2026, at 2:00 pm CT. d) Revise Seed Project SOW dated 15APR2026, as follows: - Description of work - General Requirements - 1. Demolition - 2. Mechanical - 3. Electrical - 4. Architectural - Submittals During the Contract e) Revise Contractor Bid Sheet v2; 'Subcontractors' Tab, Added Additional Subcontractor Fields (up to 50) f) Revise NCR MATOC Section L v6, as follows: - para L.3.3 -

ISO Shipping Containers - 20’ X 8’ X 8”6” CONTAINER

DLA TROOP SUPPORT

This is a Pre-Solicitation Notice of the Government’s intent to issue a solicitation for the following requirement: National Stock Number: 8150-01-463-8553 Quantity: 100 Item: ISO Shipping Containers - 20’ X 8’ X 8”6” CONTAINER Location: CONUS This is not a Request for Quote and only the Government notice of intent in the near future. When the solicitation is published on DIBBS, the Government’s requirement will be fully outlined in the solicitation. All responsible sources may submit a bid, proposal or quotation which shall be considered by the agency – REFERENCE – FAR 5.207 (c) (16).

31--BEARING,PLAIN,SELF-ALIGNING

DLA AVIATION

J041--Metasys BAS Maintenance & Support

NETWORK CONTRACT OFFICE 19 (36C259)

AN/AQS-20 Automatic Target Recognition (ATR)

NAVSEA HQ

Description The Naval Sea Systems Command (NAVSEA) intends to award a 'C' type contract with options for services, testing, technical assistance, integration, engineering service, and software support of the Automatic Target Recognition (ATR) for the AN/AQS-20 program. NAVSEA intends to award the sole source contract to Northrop Grumman Systems Corporation (NGC) in Annapolis, MD. The estimated total value of the contract, including all options, is $5,275,000.00. The period of performance is estimated to begin in July of Fiscal Year 2026 and end in Fiscal Year 2030. Background & Scope: The AN/AQS-20 systems will satisfy the U.S. Navy’s need for a wide-area reconnaissance system capable of detecting and classifying surface and near-surface moored mines. This system supports the global superiority of the U.S. Navy by countering enemy-laid mines to permit friendly maneuver or use of selected sea areas. The NGC Generalized ATR (GATR) team develops the AN/AQS-20 Multi-Forward Side Look Sonar (MFSLS) GATR interfaces. This includes the direct integration of available algorithms into the overarching GATR infrastructure. The NGC team conducts continuous development and training of the MineHunter (MH) algorithm, specifically optimized for the AN/AQS-20 Synthetic Aperture Sonar (SAS) imagery. Furthermore, the GATR team handles system integration, systems engineering, software development, tool development, and system productization to develop, test, and implement algorithm fusion capability. NGC also interfaces with parallel developments from other Government partners and Contractors to support the complete AN/AQS-20 post-mission analysis (PMA) effort. Statutory Authority and Justification for Other Than Full and Open Competition The contract will be awarded using other than full and open competition pursuant to 10 U.S.C. 3204(a)(1), as implemented by FAR 6.302-1(a)(2)(ii), “Only one responsible source and no other supplies or services will satisfy agency requirements.” Rationa