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。
Each year there is an increasing number of cars on roads and streets as millions of new cars and trucks are produced. One out of every six Americans works at putting together the parts of cars, driving trucks, building roads or filling cars and trucks with gas.  1  
Most Americans would find it hard to imagine what life would look like without cars. 2 . The polluted air becomes poisonous and dangerous to health.
 3 . That’s what several of the large car factories have been trying to do. But to build a clean car is easier said than done. Progress in this field has been slow.
Another way is to take the place of the car engine by something else.  4 . Many makers believe that it will take years to develop a practical model that pleases man.
To prevent the world from being polluted by cars, we’ll have to make some changes in the way many of us live. Americans, for example, have to cut down on the number of their total cars. They are encouraged to travel and go to work by bicycle. Bicycling is thought to help the air clean. But this change does not come easily. A large number of workers may find themselves without jobs if a car factory closes down.  5 .
Although cars have led us to a better life, they have also brought us new problems.

A．Thus the problem of air pollution would become less important than that of unemployment.

B．Americans know cars very well.

C．However, some have realized the serious problems of the air pollution that is caused by cars.

D．One way to get rid of the polluted air is to build a car that does not pollute.

E. However, the number of vehicles has continued to increase in recent years.
F. Americans won’t live without cars!
G. Inventors are now working on steam cars as well as electric cars.