On April 2, Brutus sent an offer to Caesar, to sell Caesar his personal chariot for $2,500. The offer stated that Caesar had 10 days in which to accept. Caesar received the offer on April 4. Which of the following statement(s) are true?

Select one or more: a. If Caesar sent a letter of acceptance on April 13, it would be too late to constitute a valid acceptance. b. If Brutus sent a letter of revocation, it would not be effective until received. c. Brutus must hold the offer open for the promised 10 days. d. This is an example of a firm offer.