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單項選擇題

（） can be operated independently or locked together and operated as a twin-job cranes of double capacity.

A.Two derricks
B.A derrick and a crane
C.Two cranes
D.A crane and a winch

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單項選擇題

Some cranes are mounted in pairs on a common platform which can be rotated through（）.

A.360°
B.270°
C.180°
D.90°

單項選擇題

Deck cranes require（） time to prepare for working cargo than derricks.

A.less
B.more
C.many
D.much