单选题If a reciprocating air compressor has a knock occurring in frequency with its operating RPM, the cause is probably （）.Amisalignment or worn main bearingsBinsufficient cylinder lubricationCdefective or poorly fitted valvesDall of the above
单选题
If a reciprocating air compressor has a knock occurring in frequency with its operating RPM, the cause is probably （）.
A
misalignment or worn main bearings
B
insufficient cylinder lubrication
C
defective or poorly fitted valves
D
all of the above
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