题目

从方框中选出合适的句子，填在短文的空缺处。Electricity is an important part in our life. We can't see or touch it, but it is all around us. But some people don't know that everyone should save electricity. When it is not completely dark, many street lamps (路灯) light up early in some streets of places. Some people are sleeping with the lights on. They are all wrong. Here I make some suggestions about stopping these things.1. We light them from 7:00 p.m.—5:30 a.m. in summer, and from 5:30 p.m.—6:00 a.m. in winter.2. If some small areas are very dark, just turn on a若3x4yn-2与-5xm+2y2n-8是同类项，则mn= ．