单项选择题 An operational amplifier, as used in todays consoles, has a calculated gain of 5This means that when the input changes （）

单项选择题 Electrical failures in motors are caused by the breakdown of insulation, which may be caused by （）

单项选择题 A three-phase alternator operates at 450 volts with a power factor of 0~8.If the ammeter indicates 250 amperes, what should be the KW meter reading （）.