The earth is our home. We must   41  it. Life today is easier than it was millions of years ago, but it has brought some new problems. One of the biggest　 42  is pollution.

Man has polluted the earth .The more people, the more pollution. When the land(土地) was used up or the river was not clean in a place, man went to　  43  place. Now man is slowly polluting the whole world.

Many countries don’t let people burn(燃烧) anything bad for air in houses and factories in the city. Pollution by SO2 （二氧化硫）is now the most dangerous kind of　  44  pollution. It　  45  by heavy traffic. People say it’s best to　  46  bikes. When you are riding, there is　  47  pollution. But most people don’t want to go to work by bike. It’s not because bikes are expensive　  48  the number of cars on the roads is becoming larger. So more and more people go to work by car. Then things are getting　  49  . We should have special roads only for bikes and make it　  50  difficult and expensive for drivers to take their cars into the city that they will go back to using their bikes.