请认真阅读下列短文，并根据所读内容在文章表格中的空格里填入一个最恰当的单词。注意：每个空格只填一个单词。请将答案写在答题卡上相应的横线上。
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 realised 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 that can please man .
To prevebt the world being polluted by cars , 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 doesn’t close down ---- 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.
Title : (1) _____ and Pollution in America