单项选择题

Knock ______ the door before you enter the room.

A. on
B. for
C. at
D. in
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单项选择题
It was Japan ______launched the war against China.
A. that
B. when
C. whom
D. which
单项选择题
John said that he ______ London twice.
A．have been to
B．have gone to
C．had gone to
D．had been to
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