Builder and Owner entered into a valid written contract under which Builder agreed to erect a house according to certain plans and specifications, for which Owner agreed to pay the sum of $200,000, upon completion. During the course of construction, building costs increased significantly. Builder informed Owner of the increased costs, and the parties agreed in writing that Builder could omit installing the air conditioning unit called for by the specifications (thus saving Builder approximately $10,000) and nevertheless receive the full construction contract price. Under the general rule, this subsequent written agreement is: Enforceable, because an agreement modifying a contract for the sale of goods (the air conditioning unit) needs no consideration to be binding. O Enforceable, on the theory that Builder gave up his right to breach the contract (walking off the job and refusing to complete the building) in reliance on Owner's agreement to the modification. Enforceable as a novation, which superseded the original construction contract Unenforceable for lack of consideration, even though in writing