【名词&注释】
机动性(mobility)、针对性(pertinence)、特殊情况(special case)、冗余度、操纵性(maneuverability)、信息化、智能化
[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