Under a Contract with Valley Vineyard, Walsh Begins Grading a Terraced

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. Their agreement is

A) unenforceable because Walsh's performance was a preexisting duty.
B) unenforceable because Valley's promise was illusory.
C) enforceable.
D) unenforceable because Walsh's request modified the contract.