Baker and Able Signed a Contract Which Required Able to Purchase

Baker and Able signed a contract which required Able to purchase 600 books from Baker at 90¢ per book.Subsequently,Able,in good faith,requested that the price of the books be reduced to 80¢.Under the circumstances,the oral agreement is:

A) unenforceable,because Able failed to give consideration,but proof of it will be otherwise admissible into evidence.
B) unenforceable,due to the statute of frauds,and proof of it will be inadmissible into evidence.
C) enforceable,but it is not a valid agreement.
D) enforceable,and proof of it will be admissible into evidence.