If an Offer Is Made by Mail and an Acceptance

If an offer is made by mail and an acceptance by mail is requested, when does a letter revoking the previous offer actually take effect?

A) Never. A revocation must be done in person.
B) At the time that it is received, if before acceptance
C) At any time before actual performance of the contract has begun
D) At any time before a letter of acceptance is received by the offeror
E) At the time that it is posted