电动机的机械特性是指电动机的（）之间的关系。
A.功率与转速
B.功率与转矩
C.转矩与转速
参考答案：C
My brother usually ___ his classroom after school. But now he ___ basketball.
A．cleans; plays
B．is cleaning; plays
C．cleans; is playing
D．is cleaning; is playing
下列不属于直流电机转子构成的是（）。
A.电枢铁芯
B.电枢绕组
C.换向极
D.换向器
