Shane sells books wholesale to retailers. Brenda is a retailer. On February 1, by signed writing, Brenda and Shane form a contract under which Shane will sell Brenda 500 copies of the novel Snowmaker, to be delivered on March 1, Brenda to pay a total purchase price of $750. On February 3, Brenda contacts Shane by signed writing: "Our contract calls for you to deliver 500 copies of Snowmaker on March 1. We'd like to have an additional twenty-five copies, 525 in all for the same $750 total, all to be delivered by February 15. Agreed?" Shane writes back, "Agreed." As of February 3, Shane's contractual obligation is to deliver
A.500 copies of the book by March 1.
B.500 copies of the book by February 15.
C.525 copies of the book by March 1.
D.525 copies of the book by February 15.