Which of the listed pumps is NOT a positive displacement pump?（）

A.a high suction head
B.rotating rings
C.badly worn bearings
D.a loose stuffing box gland

A.solenoid valve to seize
B.compressor to hunt
C.formation of flash gas in the liquid line
D.expansion valve to freeze open

A.insufficient condenser cooling water
B.excessive condenser cooling water
C.air in the refrigeration system
D.excessive refrigerant in the system