风机起动前检查，应保证电压在额定电压的±5％范围内。
参考答案：对
直流电动机的工作原理是[ ]
A．通电导体的周围存在着磁场
B．通电导体在磁场里受到力的作用
C．电磁铁通电后，会吸引铁片
D．电磁感应
Janny ____ have kept her word. I wonder why she changed her mind. [ ]
A. must
B. should
C. need
D. would
