Under a contract with Soy Farms, Tai begins grading a terraced hillside for planting. Halfway through the project, Tai 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 Tai$$s request modified the contract.

b$.$ enforceable.

c$.$ unenforceable because Tai$$s performance was a preexisting duty.

d$.$ unenforceable because the farm$$s promise was illusory.