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B is corrent. In general, an offer remains effective until it is withdrawn, rejected, or accepted. 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, in the instance in which the offeree sends a rejection first and then an acceptance, the acceptance is not effective until received. In this case a contract was created because the July 6 telegram accepting the offer was received before the July 5 rejection letter.
A is incorrect. An offeror can always withdraw his offer unless prohibited from doing so because of the existence of an option or the firm offer rule. An option is not present because Mix did not give consideration for the opportunity to purchase the building. The firm offer rule does not apply because this contract does not involve the sale of goods.
C is incorrect. While an effective rejection does terminate an offer, a rejection is never effective until communicated.
D is incorrect. The firm offer rule is not applicable to the sale of real property.