交流电动机调速方法有：（1）变极调速、（2）变压调速、（3）变频变压调速、（4）变转差率调速。()
参考答案：对
根据情景选择合适的句子。
1. 你到水果店里去买水果，想知道有没有香蕉，可以问：[ ]
A. How much are they?
B. Do you have any bananas?
2. 想知道梨卖多少钱，可以说：[ ]
A. Two yuan.
B. How much are the pears?
3. 觉得西瓜的价钱有点贵，可以说： [ ]
A. That's expensive.
B. I'll take two, please.
4. 想知道这些铅笔是在廉价出售的吗，可以问：[ ]
A. How much is this?
B. Are the pencils on sale?
42-14÷7先算______，再算______．
