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
单项选择题 Some cranes are mounted in pairs on a common platform which can be rotated through（）.
单项选择题 Deck cranes require（） time to prepare for working cargo than derricks.
单项选择题 The duty of a deck winch is to lift and lower a load by means of a fixed rope on （）.
