Johnson Bakery agrees to supply Higgen’s Restaurant with all the bread that it requires for one year. When a shortage causes the price of wheat to rise sharply, Johnson can continue supplying bread only at a much higher price. The parties agree to modify the contract so that the buyer will pay a higher price. The change is

a. enforceable as long a the parties voluntarily agreed to the modification.
b. uneforceable due to the preexisting duty rule
c. unenforceable because Johnson is taking advantage of a shortage to boost profits
d. unenforceable because a valid contract already exists