Shane Contracts to Build a Garage for Bob for a Price

Shane contracts to build a garage for Bob for a price of $6,000.Because of an increase in the cost of labor and materials, Shane refuses to perform.Bob wants the garage, so he agrees to pay an additional $500.In this case:

A) Bob must pay the additional $500.
B) Shane has given no additional consideration, and under the common law he must perform at the agreed-upon original price.
C) the substitute agreement is an illusory contract.
D) the debt is a disputed one, and therefore Bob is obligated to pay the additional money.