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單項(xiàng)選擇題

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

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

單項(xiàng)選擇題

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