下述（）不是计算天然建材储量的方法。 A.分层总和法  B.平行断面法  C.平均厚

阅读下面短文，根据所读内容在文后第53至第57小题的空格里填上适当的单词或短语，并将答案转写到答题卡上。
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

Problem	Methods	Disadvantages	57.____________
53.______________	Building a new kind of clean car.	Progress in this field is slow.	Americans may live a happy but sad life.
		It takes years to develop a 55._______
	54.________ on the number of cars	Many workers may 56.__________
	Traveling and going to work by bike.