Randy Asked Carl to Detail His Brand-New Sports Car by Painting

Randy asked Carl to detail his brand-new sports car by painting flames on the side of the vehicle.After the parties signed a contract and after Carl had already started to paint,Randy decided that he wanted Carl to also install a new stereo system in the car.Carl sued Randy for modifying the existing contract without any prior intimation; but he lost the lawsuit because:

A) Randy's act was unenforceable for lack of consideration.
B) Randy's promise was enforceable.
C) The old contract automatically got cancelled with the addition of a new condition.
D) Carl's act was not a preexisting duty and hence of no legal value.