Americans won't live without cars! Each year there is an increasing number of cars on roads and streets, as millions of new cars are produced. One out of every six Americans work at putting together the parts of cars, driving trucks, building roads or filling cars with gas. 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 and dangerous to health. One way to get rid of the polluted air is to design a new car that does not pollute. That's what several large car factories are trying to do. But to build the new car is easier said than done. Progress in this field has been slow. Another way is to take place of the car engine 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 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 the number of their cars they are encouraged to travel and go to work by bike. Riding a bike is thought to help keep the air clean. But this change does not come easily, a large number of workers may find themselves with no jobs if a car factory closes down. And the problem of air pollution would become less important than that of unemployment. Large car factories are trying to build a clean car,  _
which is clean itself which does not pollute the air which is used to clean streets which is easier to make
Question: Americans won't live without cars! Each year there is an increasing number of cars on roads and streets, as millions of new cars are produced. One out of every six Americans work at putting together the parts of cars, driving trucks, building roads or filling cars with gas. 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 and dangerous to health. One way to get rid of the polluted air is to design a new car that does not pollute. That's what several large car factories are trying to do. But to build the new car is easier said than done. Progress in this field has been slow. Another way is to take place of the car engine 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 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 the number of their cars they are encouraged to travel and go to work by bike. Riding a bike is thought to help keep the air clean. But this change does not come easily, a large number of workers may find themselves with no jobs if a car factory closes down. And the problem of air pollution would become less important than that of unemployment. Large car factories are trying to build a clean car,  _
A. which is clean itself
B. which does not pollute the air
C. which is used to clean streets
D. which is easier to make
Answer: