Elmer Wrote a Letter to His Friend Fred Offering to Sell

Elmer wrote a letter to his friend Fred offering to sell Fred an 80-acre farm for $200,000. After mailing the letter, Elmer learns that the farm is actually worth $300,000 and changes his mind about selling. In this case, Elmer:

A) has made a firm offer to Fred which cannot be revoked.
B) can revoke his offer at any time before Fred accepts it, because there is no consideration to keep it open.
C) must keep the offer open, because this is an option contract.
D) cannot, because of promissory estoppel, revoke his offer to Fred.