单项选择题 When two different metals are joined to form a closed circuit and exposed to different temperatures at their junctions （） which can be used to measure temperature.

单项选择题 （） is provided to alternators, in the event of prime mover failure to ensure that the alternator does not act as a motor.

单项选择题 To parallel an incoming machine to a running machine therefore it is necessary to ensure firstly that （）.