单项选择题

Which of the following materials is a good insulator?（）

A.steel
B.aluminum
C.glass
D.copper

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To protect the rotor of a motor disassembled for maintenance or overhaul, it should be（）

A.suspended by wire slings in one comer of the shop
B.wrapped in several layers of heavy paper or carboard
C.supported by flat wood blocks on the workbench
D.returned to the frame as soon as the bearings are removed

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When paralleled, AC generators must have the same （）

A.frequency
B.number of phases
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D.All of the above

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A DC generator is used to supply direct current in order to maintain an AC generator field and is known as a/an （）

A.rotor
B.stator
C.exciter
D.armature