单项选择题
A reciprocating air compressor is running roughly and vibrating excessively, indicating that the （）.
A.compressor is overloaded
B.motor is overloaded
C.foundation bolts are loose
D.belts are too tight
单项选择题 The term "pour point" is defined as the lowest temperature at which lubricating oils will flow （）.
单项选择题 When liquid reaches the compressor of a refrigeration system through the suction line, the condition is called （）.
单项选择题 What is a function of the wearing rings used in most centrifugal pumps?（）