根据短文理解, 完成下了各题。
Each year there is an increasing number of cars on the 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 the cars, driving trucks, building roads, or filling car with gas. Americans won't live without cars!
Most Americans would find it hard to think what life could look like without a car. However, some have realized the serious problem of the air pollution caused by the car. The polluted air becomes poisonous and dangerous to health.
One way to get rid of the polluted car is to design a car that does not pollute. 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 car engine （发动机）by something else. Inventors are now working on steam cars as well as electric cars. Many makers believe that it will take years to develop a practical model that pleases man.
To prevent the world from being polluted by car, we'll have to make some changes. Americans, for example, have to cut down on the number of their total cars. They are encouraged to travel and to go to work by bicycle. Bicycling is thought to help keep the air clean.
But this change doesn't come easily—a large number of workers may find themselves without jobs if a car factory closes down. Thus the problem of air pollution would become less important than that of unemployment.
Although cars have led us to a better life, they have also brought us new problems.