In the refrigeration system, a shortage of refrigerant is indicated by （）.

单项选择题Which of the following conditions must exist for heat to flow from one object to another?（）

A.The two objects must be in physical contact
B.The two objects must be the same size
C.There must be an existing temperature differential
D.There must be an existing weight differential

单项选择题A warmer than normal compressor suction line might be caused by （）.

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 （）.

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?（）

A.Radiation
B.Conduction
C.Convection
D.Each of the above

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?（）

A.Shortage of refrigerant oil
B.Slight shortage of refrigerant
C.Excessive condenser cooling water flow
D.Faulty expansion valve