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

a$.$

unenforceable because Red$$s promise was illusory.

b$.$

enforceable on its face.

c$.$

unenforceable because Sancho$$s request modified the contract.

d$.$

unenforceable because Sancho$$s performance was a preexisting duty.