根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。
The earth is our home. We must take care of it. Life today is easier than it was hundreds of years ago, but it has brought some new problems. . We can see it, smell it, drink it and even hear it. Man has polluted the earth. The more people, the more pollution. , the problem was not so serious. When the land was used up, the river was not clean in a place, man went to another place. Now man is slowly polluting the whole world. , It's bad for living things in the world. Many countries don't let burn something bad for air in houses and factories in the city. Pollution by SO2 is now the most dangerous kind of air pollution. It's caused by heavy traffic. . 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 put away their bikes and go to work by cars, then things getting worse and worse. and make it so difficult and expensive for drivers to take their cars into the city that they will go back to use their bikes.
A. For industry was not well-developed.
B. We should have special roads only for bikes
C. Air pollution is still the most serious
D. Because there were not so many people many years ago
E. People say it's best to ride bikes
F. One of the biggest problems is pollution