单项选择题
The nameplate of an AC non-synchronous motor shows that running speed is 1460r/minfrequency 50HzYou can infer that the number of magnetic poles is（）
A.2
B.4
C.6
D.8
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With one or more cylinders out of operation, the engine speed must be reduced because（）
A.in such cases, the air supply is no longer optimal, the thermal load will be higher
B.governor hunting will occur
C.torsional vibrations as other mechanical vibrations may occur
D.all of the above are correct -
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A few spots of turbocharger can be caused by（） ①crash stop of engine ② rapid increased of engine speed
A.①only is correct
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A.improve power factor
B.reduce voltage oscillation
C.increase the insulation of circuits
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