单项选择题
A.insufficient lubrication
B.insufficient refrigerant
C.excess refrigerant
D.excessive opening of the expansion valve
单项选择题 The unloading system on an air compressor will （）.
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?（）