题目

 — How is the smog control going on in our capital city? — It couldn’t be worse. You ________ believe Beijing upgraded the smog alert from yellow to orange and then red again yesterday. A. won’t  B. mustn’t  C. needn’t  D. shouldn’t 答案：A 下图为直流电动机模型．直流电动机的工作原理是________．电动机工作时，能够使线圈平稳、连续不停地转动下去是靠________来实现的．