PWM调速系统的特点是（）。
A、频带宽
B、电流脉动小，电源功率数高
C、动态硬度好
D、ABC都正确
参考答案：D
，Mom had a cup of coffee and a few minutes’ rest．
A．With her housework done
B．With her housework being done
C．With her housework doing
D．With her housework to do
天王补心丹最适宜于治疗郁病的
A．心脾两虚证B．肝阴亏虚证C．心阴亏虚证D．心神惑乱证E．肝气郁结证
