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單項(xiàng)選擇題

A fire starts when the following （） factors come together.

A.combustible material
B.ignition source
C.oxygen
D.A and B and C

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單項(xiàng)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

單項(xiàng)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°