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單項(xiàng)選擇題

（） is the production of pure water from sea water by evaporation and re-condensing.

A.Evaporation
B.Circulation
C.Condensation
D.Distillation

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單項(xiàng)選擇題

Which of the following can be used in hydraulic transmission system?（）

A.Hydraulic oil
B.Animal oil
C.Light oil
D.Fuel oil

單項(xiàng)選擇題

（） 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