[单项选择]

B

Each year there is an increasing number of cars on roads and street are millions of new cars are produced. One out of every six Americans works at putting together the parts of cars, driving trucks, building roads of filling cars with gas. Americans won’t live without cars!
Most Americans find it hard to think what life would be without a car. However some have realized the serious problem of air pollution by cars. The polluted air is poisonous(有毒的) and dangerous to health.
One way to get rid of(排除)the polluted air is to build a new kind of car. That’s what several of the large car factories are 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(发动机) by something else. Engineers are now working on some new cars. Many makers believe that it will take years to develop a practical model that pleases man.
To prevent the
A. is clean in itself
B. is used to clean streets
C. does not pollute the air
D. is easier to make