如图，DE是△ABC的中位线，若BC的长为3cm，则DE的长是（ ）

世界环境日是（ ）

    I'm living in a busy city. There are too many people and too many cars. Too many people and too many cars cause too crowded traffics. At the same time, the cars gives out much smog (烟雾), especially (尤其是) at rush hours (高峰期). The smog pollutes (污染) the city terribly. The problem is becoming more and more serious.

We should do something to prevent it.

    First, I think we should have more police officers at the busy crossings. They can stop the traffic jams (阻塞). Second, we should not make more parking garages (停车位) for cars. When people think that it's easy to park in cities, more people will drive to work and it will make the traffic more crowded. Third, we should make it more expensive for drivers to drive ears to cities. Then more people will leave their cars for their bikes. Fourth, we should encourage people to take a bus to work. Fifth, we should build special ways for bikes so that more people will go to work by bike instead of by bus or in a car. Finally, we should spend more on the public transportation(公共交通.). For example, we can build more underground lines. Of course, we can also do many other things to improve our traffic.