On March 1 Susan Offers to Mow Sam's Yard Every

On March 1 Susan offers to mow Sam's yard every week through the summer for $35.Sam says that he wants to think about it.Susan says that she will leave the offer open only until March 15.On March 15 Sam drops a letter in the mailbox telling Susan that he accepts her offer.Susan does not receive the acceptance until March 17.On March 16,not having heard from Sam,Susan decides that she is out of the mowing business and sells her mower.When she gets Sam's acceptance,she calls him up and tells him to forget it,Sam did not accept in time,and that she is not mowing.Sam says they have a contract.Who is right under the mailbox rule?

A) Susan is right, because Sam's acceptance was not effective until she received it after March 15.
B) Susan is right, but only because she had sold the mower and had no means by which to perform.
C) Sam is right, because his acceptance was effective when he placed it in the mailbox.
D) Sam is right, because he was unaware that Susan had sold the mower when he placed the acceptance in the mailbox.
E) Susan is right, because Sam should have called her by telephone.