When paralleling two AC generators, the frequency of the machine coming on－line, immediate

When paralleling two AC generators, the frequency of the machine coming on-line, immediately prior to closing its breaker, should be _____.

A.slightly less than the oncoming generator frequency

B.the same as the bus frequency

C.slightly greater than the bus frequency

D.the same as the bus voltage

请帮忙给出正确答案和分析，谢谢！