单项选择题
A.increase compressor discharge pressure on demand
B.increase compressor operating speed as necessary
C.allow the motor to turn the compressor opposed only by friction
D.reduce the compressor frictional load when starting
单项选择题 Which of the listed methods of heat transfer takes place when two substances of different temperatures are in physical contact with each other?（）
单项选择题 Which of the conditions listed will cause a refrigeration compressor to run constantly without simultaneously decreasing the temperature in the refrigerated space?（）
单项选择题 Air compressor cylinder un-loaders enable the compressor to （）.