单项选择题
A.noise and vibration
B.an increase in discharge pressure
C.an increase in suction pressure
D.lifting of the relief valve
单项选择题 One cause of high head pressure occurring in a refrigeration system can be （）.
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 （）.