On Thursday, Angie decided to accept Harry's counteroffer for the purchase of his house, signed the purchase agreement, and placed it in the mail. On Friday, Harry decided that he had asked too low a price for the house, and faxed Angie stating that the offer was withdrawn. On Saturday, Harry received her acceptance in the mail. Under the mailbox rule, which of the following is true?
a. Harry withdrew his offer, so no contract is formed
b. Harry did not withdraw his counteroffer in time, so a contract is formed
c. Harry may not make a counteroffer
d. Harry's withdrawal was ineffective since it was not in writing