Under a contract with Valley Vineyard, Walsh begins grading a terraced hillside for the planting of grapes. Halfway through the project, Walsh asks for $$5,000$ over the contract price, claiming an increase in the "cost of doing business." Valley agrees but later refuses to pay. Valley's agreement to pay more is

unenforceable because Walsh's performance was a preexisting duty.

unenforceable because Valley's promise was illusory.

enforceable.

unenforceable because Walsh's request modified the contract.