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. a. Elmer has made a firm offer to Fred which cannot be revoked. b. Elmer can revoke his offer at any time before acceptance, there is no consideration to keep the offer open. c. Elmer must keep the offer open, because this is an option contract. d. Elmer is prohibited from revoking his offer to Fred under the doctrine of promissory est