单项选择题
Which of the listed precautions should be taken when cleaning the internals of a motor with compressed air?（）
A.Open the machine on both ends so as to allow the air and dust to escape B.Be certain that the circuit breaker is opened and tagged on the feeder panel C.Be certain that the air is clean and as dry as possible D.All of the above
单项选择题 Since fuse elements are made of zinc or any alloy of tin and lead, the melting point of the fuse element must be （）.
单项选择题 The most common source of excitation for synchronous motors is a/an （）.
单项选择题 Before reassembling any machinery, you should （）.
