单项选择题
A.insufficient cooling water flow to the condenser
B.a low refrigerant charge in the system
C.the liquid valve is open too much
D.high evaporator superheat
单项选择题 The control mode in which the final control element is moved from one of two flexed positions to the other is known as （）.
单项选择题 Pitting in the suction areas of a centrifugal pump bronze impeller is usually caused by （）.
单项选择题 If the head pressure of a reciprocating refrigeration compressor is excessive, （）.