换向器在直流发电机中起（）的作用。
A、交流电变直流电
B、直流电变交流电
C、保护电刷
D、产生转子磁通
参考答案：A
He is _____ tearing up what he has written and beginning over again.
A．simply
B．constantly
C．gradually
D．generally
代数式4-a2+2ab-b2的最大值是_________ 当取最大值是时，a与b的关系是_________
