Which of the Following Is Correct with Respect to the Operation

Which of the following is correct with respect to the operation of the postbox rule?

A) For the postbox rule to apply, the offer must be made by mail.
B) If the postbox rule applies, an acceptance must be communicated and heard by the offeree before it is effective.
C) If the postbox rule applies, the acceptance is effective at the point of mailing.
D) If the postbox rule applies, the only way for an acceptance to be effective is to send it by mail.
E) For the postbox rule to apply, the offeror must make that clear when the offer is made.