Under a contract with River Valley Farm, Sancho begins grading a terraced hillside for planting. Halfway through the project, Sancho asks for $5,000 over the contract price, claiming an increase in the cost of doing business. The farm agrees but later refuses to pay. The agreement to pay more is:

a) unenforceable because Sanchos performance was a preexisting duty

b) unenforceable because the farms promise was illusory

c) enforceable

d) unenforceable because Sanchos request modified the contract