单项选择题
Complete maintenance of electrical motors should include periodic checks for machine （）
A.vibration B.watertight integrity C.speed droop D.reactive power
单项选择题 The frequency of an alternator is controlled from the main switchboard by adjusting the（）
单项选择题 The frequency of an AC generator is controlled by the （）
单项选择题 Capacitance is the property of an electric circuit opposing a change in the （）
