单项选择题
A.Reverse current protection
B.Reverse power protection
C.Synchronising device
D.Frequency meter
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 （）.
单项选择题 An a-cswitchboard is usually（） the corresponding d-cboard because of the higher voltage and lower currents and smaller bus-bars.