单项选择题
A.a current will flow
B.a power will develop
C.a heat will flow
D.a pressure will produce
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 （）.
单项选择题 The magnetic field is provided by electromagnets so arranged that adjacent poles have （）.