单项选择题
A.Special tools are required for lapping or grinding
B.It is impossible to throttle flow with a butterfly valve
C.To close the valve, it is only necessary to turn the handle a quarter of a turn
D.The butterfly valve should never be used in a freshwater system
单项选择题 The simplest method to use for determining if a centrifugal pump is operating as designed, is to （）.
单项选择题 Air compressors are equipped with unloading systems for the purpose of（）.
单项选择题 When a refrigeration compressor motor fails to start, the FIRST thing that should be checked for is a （）.