直流电机安装换向极的主要作用是（）。
A．提高发电机的输出电压
B．增大电动机的电磁转矩
C．减少换向时的火花
D．减小电动机的电磁转矩
参考答案：C
根据句意填写单词。
1. The box is _ . You can put your books in it.
2. I'm sorry for being late, Mr Smith. I worked too late last night and o _ this morning.
3. It's _ of you to believe him. He never tells the truth.
4. We all know that comedies always have very good and surprising e
5. On April Fool's Day we can hear many different kinds of h
二氧化碳浸渍法
