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B is corrent. Under the mailbox rule, an acceptance is ordinarily effective when sent if transmitted by the means authorized by the offeror, or by the same means used to transmit the offer if no means was authorized. However, the offeror may stipulate that acceptance is effective only when received by the offeror. If the offeror does stipulate this, the mailbox rule does not apply.
A is incorrect. The fact that both the offeror and offeree are merchants is not relevant in determining when the acceptance becomes effective.
C is incorrect. The length of the offer does not affect when acceptance is effective (i.e., the mailbox rule would apply).
D is incorrect. The type of offer does not affect when acceptance is effective (i.e., the mailbox rule would apply).