单项选择题

By which of the following means is force efficiently provided to vary the pitch of the blades on a modem controllable pitch propeller?（）

A.Hydraulic
B.Mechanical
C.Pneumatic
D.Electrical

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单项选择题

Which of the following statements concerning energy is correct?（）

A.Energy can be created or destroyed
B.Energy may not be transformed
C.The total quantity of energy in the universe is always the same
D.None of the above

单项选择题

In a compression refrigeration cycle, the temperature of the liquid refrigerant experiences its greatest decrease in the（）.

A.evaporator
B.compressor
C.expansion valve
D.condenser

单项选择题

Excessive oil foaming in the crankcase of a refrigeration compressor is most likely to occur（） when the compressor.

A.has continuously for a long period
B.suction pressure is below normal
C.oil level is below normal
D.starts after a long idle period