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

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

单项选择题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

单项选择题On which of the following heated surfaces of a flash type evaporator would you be more likely to find soft scale formation?（）

A.Feed-water heater internal tube surfaces
B.Internal distillate cooler tubes
C.Flash chamber vertical surfaces
D.Distilling condenser tubes