Walter begins pouring cement for a driveway on Mark's property under a contract between the parties. Halfway through the job, Walter asks for $3,000 more over the contract price, arguing that the price of cement has increased. Mark agrees but later refuses to pay. His promise to pay the extra money will likely be

a. unenforceable because it is illegal.

b. enforceable because Mark bears the risk if the price of cement increases.

c. unenforceable because Walter's performance was a preexisting duty under the contract.

d. enforceable because $3,000 is not so much money.