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

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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

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Strainers are commonly used in hydraulic systems to （）.

A.protect the pump from fine soluble contaminants
B.prevent solid particles from entering the pump
C.prevent solid particles from entering the filter
D.protect the directional control valves