On April 6, Apple entered into a signed contract with Bean, by which Apple was to sell Bean an antique automobile having a fair market value of $150,000, for $75,000. Apple believed the auto was worth only $75,000. Unknown to either party the auto had been destroyed by fire on April 4. If Bean sues Apple for breach of contract, Apple's best defense is
a. unconscionability
b. risk of loss had passed to bean
c. lack of adequate consideration
d. mutual mistake