阅读下面短文，根据所读内容在文后小题的空格里填上适当的单词或短语。注意：每空不超过3个单词。 Each year there is an increasing number of cars as millions of new cars are produced in America. Americans will not live without cars! However, some have realized the serious problem of air pollution by cars. It is necessary to find ways to solve the problem of air pollution. One way to clean the air is to build a new kind of clean 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 by something else. Engineers are now working on it. Many makers believe that it will take years to develop a practical model for us. To prevent the world being polluted by cars, Americans have to make some changes in the way of their life. They have to cut down on the number of their cars and are encouraged to travel and go to work by bike. But this change does not come easily. Many workers may find themselves without jobs if a car factory closes down. And the problem of their pollution would become less important than that of unemployment. Americans may live a happy but sad life for a long time because of the car problem. Cars and Pollution in American
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