单项选择题
（） play a very important part in hydraulic systems by preventing interaction between different parts of the hydraulic circuit.
A.Motors B.Check valves C.Safety valves D.Reducing valve
单项选择题 Cargo handling unit is rated according to the （） to be lifted.
单项选择题 When the union purchase derrick is operated, one derrick is positioned over the quayside and the other almost vertically over the （）.
单项选择题 In a general cargo crane, （） is fitted to lift the cargo load.
