问题
单项选择题
Exchange of answerbacks act s a confirmation that the messages（）at the destination.
A.have received
B.have been received
C.are received
D.are being received
答案
参考答案：B
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Exchange of answerbacks act s a confirmation that the messages（）at the destination.
A.have received
B.have been received
C.are received
D.are being received
参考答案：B