直流电机中的电刷是为了引导电流，在实际应用中一般都采用（）。
A、铜质电刷
B、银质电刷
C、金属石墨电刷
D、电化石墨电刷
参考答案：D
He found himself by a crowd of excited people and to escape.
A．surrounded; trying
B．surrounding; trying
C．surrounding; tried
D．surrounded; tried
肾衰的病人往往缺钙，这是因为肾不能产生（）
A.肾素
B.血管紧张素
C.醛固酮
D.活性维生素D3
