单项选择题

An accidental path of low resistance, allowing passage of abnormal amount of current is known as a/an（）.

A.open circuit
B.short circuit
C.polarized ground
D.ground reference point

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Which of the following statements represents the main difference between a relay and a contactor?（）

A.Contactors control current and relays control voltage
B.A relay is series connected and a contactor is parallel connected
C.Contactors can handle heavier loads than relays
D.Contactors are made from silver and relays are made from copper

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Any electric motor can be constructed to be（）.

A.short proof
B.ground proof
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Current changing in direction and rising and falling in value is （）.

A.AC
B.DC
C.generator
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