单项选择题
Which of the listed pumps is NOT a positive displacement pump?（）
A.Centrifugal B.Reciprocating C.Gear D.Rotary
单项选择题 Heavy wear in a particular area of the inner circumference of a packing ring may be caused by （）.
单项选择题 A pressure drop through the refrigerant liquid line may cause （）.
单项选择题 Low compressor head pressure in a refrigeration system can be caused by（）.
