Which of the Following Statements Best Describes the Mailbox Rule

Which of the following statements best describes the mailbox rule?

A) Unless otherwise agreed, an acceptance is effective when it is sent if it is sent correctly.
B) If no time is specified in the offer, then the law will impose a reasonable time based on the circumstances.
C) A change in the offer by offeree will only be considered a counteroffer if the offeree intends it to be so.
D) If an offer is made but before it can be accepted the subject matter of the offer becomes illegal, the offer is terminated.