单选题Electrical failures in motors are caused by the breakdown of insulation, which may be caused by （）Apenetration of moistureBaccumulation of dirtCoverheatingDAll of the above
单选题
Electrical failures in motors are caused by the breakdown of insulation, which may be caused by （）
A
penetration of moisture
B
accumulation of dirt
C
overheating
D
All of the above
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