John Is the Project Manager for a Large Project in His

John is the project manager for a large project in his organization. A new change request has been proposed that will affect several areas of the project. One area of the project change impact is on work that a vendor has already completed. The vendor is refusing to make the changes as they've already completed the project work they were contracted to do. What can John do in this instance?

A) Withhold the vendor's payments until the issue is resolved.
B) refer to the contract agreement for direction.
C) Refer the vendor to the Service Level Agreement (SLA) and insist that they make the changes.
D) Review the Request for proposal (RFP) for guidance.