The earth is our home. We must take care of it. Life today is easier than it was _________ years ago，but it has brought some new problems. One of the biggest problems is the pollution. We can see it，smell it，drink it and even hear it.

Man has polluted the ____________. The more people，the more pollution. Many years ago，the problem was not so serious because there were not so____________ people. When the land was used up or the river was not clean in a place, man went to ____________ place. Now man is slowly polluting the whole world.

____________ pollution is still the most serious. It's bad for ____________ things in the world.

Many countries don't ____________ people burn anything bad for air in houses and factories in the city. Pollution by SO2is now the most dangerous kind of air pollution. It's caused by heavy traffic. People say it's _________ to ride bikes. When you are riding, there is no pollution. But even in developed countries, most people don't go to work by bike. It's not because bikes are expensive or people are tired if they ride to work. It's because the number of cars on the roads becomes larger. So more people ____________ their bikes and go to work by car, and then things are getting worse and worse. We should have special roads only for bikes and make it ____________ easy for people that they will go back to use their bikes.