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 pollution（污染）. We can see it, smell it, drink it and even       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 (用光), 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 le t people burn       for air in houses and factories in the city. Pollution by SO2 is now the most dangerous kind of       pollution. It’s caused by heavy traffic. People say it’s       to ride bikes. When you are       , there is no pollution. But even in developed countries, most people don’t go to work by bike. It’s not      bikes are expen sive 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 cars,  then things are getting       . We should have special roads only for bikes and make it       difficult and expensive for drivers to take their cars into the city that they will go back to use their bikes. 

1.A. hundred       B. hundreds of      C. hundred of      D. hundreds 

2.A. listen         B. listen to         C. hear of        D. hear 

3.A. moon          B. star               C. earth             D. sun 

4.A. lot            B. little             C. many             D. much  

5.A. others        B. the others       C. the other        D. another  

6.A. Air           B. Food              C. Water            D. Noise  

7.A. life           B. live              C. living            D. lives 

8.A. something bad              B. bad something  

C. something good              D. good something 

9.A. water           B. air             C. noise          D. white 

10.A. least           B. best            C. most             D. worst 

11.A. driving       B. biked           C. riding           D. ridden 

12.A. why             B. because          C. what              D. how 

13.A. put on          B. look at         C. put away        D. look up 

14.A. better and better              B. more and more  

C. worse and worse                     D. bigger and bigger 

15.A. quite            B. such              C. very               D. so