单项选择题
Which of the following materials is a good insulator?（）
A.steel B.aluminum C.glass D.copper
单项选择题 To protect the rotor of a motor disassembled for maintenance or overhaul, it should be（）
单项选择题 When paralleled, AC generators must have the same （）
单项选择题 A DC generator is used to supply direct current in order to maintain an AC generator field and is known as a/an （）
