單項(xiàng)選擇題

You should keep national flag（）.

A.flying
B.flew
C.flown
D.flies

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You should slack the mooring lines （）when the wind is big.

A. at the same time
B. on time
C. in time
D. at times

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We must （）to inspect the problems about the safety of fire prevention.

A.prepares
B.preparing
C.prepared
D.prepare