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單項選擇題

The bottom of the mast rests on a part of the keel called the mast（）.

A.foot
B.heel
C.step
D.sole

單項選擇題

The boom stops are installed on an offshore crane to（）.

A.prevent the boom from being raised too high
B.prevent the boom from swinging
C.support the boom when not in use
D.prevent the boom from being lowered