單項選擇題

（）we stand away from the church bell, it takes time for the sound waves to reach us.

A.If
B.Whenever
C.Because
D.Where

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單項選擇題

Before（）, a station shall take precautions to ensure that its emission will not interfere with transmission already in progress.

A.transmit
B.to transmitting
C.transmitting
D.in transmitting

單項選擇題

Two vessels（）while they „re sailing in Huangpu River.

A.strike
B.strikes
C.are striking
D.struck