单项选择题
A.Hydraulic oil B.Animal oil C.Light oil D.Fuel oil
（） play a very important part in hydraulic systems by p...
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
Cargo handling unit is rated according to the （） to be ...
单项选择题Cargo handling unit is rated according to the （） to be lifted.
A.cargo crane B.jib C.safe working load D.oil pressure
When the union purchase derrick is operated, one derric...
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 （）.
A.dock B.hold C.pedestal D.machinery space
