设备或电路发生障碍后，在较短时间内自行恢复，而障碍原因不明称为设备和电路障碍。
参考答案：错
工程队修一条路，开工9天修了270m，剩下630m．照这样计算，修完这条路共要多少天？
用单词的适当形式填空。
1. My aunt _____ (teach) me to play the violin when I was a child.
2. Look! The people in the river ______ (swim).
3. Tree-planting Day is on the ______ (twelve) of March every year.
4. Stephen spends about half an hour ______ (swim) every Saturday afternoon.
5. I don't like travelling by train. I think it's ______ (safe) to travel by train.
6. —I'm tired.
—So am I. We'd better stop ______ (walk) and have a rest.
7. Plants need CO2 when they grow, so they can make CO2 ______ (little) in the air.
