单项选择题
A.combustible material B.ignition source C.oxygen D.A and B and C
（） can be operated independently or locked together and...
单项选择题（） 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
Some cranes are mounted in pairs on a common platform w...
单项选择题Some cranes are mounted in pairs on a common platform which can be rotated through（）.
A.360° B.270° C.180° D.90°
Deck cranes require（） time to prepare for working cargo...
单项选择题Deck cranes require（） time to prepare for working cargo than derricks.
A.less B.more C.many D.much
