American cities are similar to other cities around the world: In every country, cities reflect（反映）the values of the culture. American cities are changing, just as American society is changing.

After World War Ⅱ, the population of most large American cities became smaller; however, the population shifts（转移）to and from the city reflect the changing values of American society. In the late 1940s and early 1950s, city residents（居民）became richer. They had more children so they needed more space. They moved out of their flats in the city to buy their own homes. They bought houses in the suburbs（郊区）.

Now things are changing. The children of the people who left the cities in the 1950s are now adults. Many, unlike their parents, want to live in the cities. They continue to move to Sun Belt cities and older ones of the Northeast and Midwest. Many young people are moving back into the city. They prefer the city to the suburbs because their jobs are there; or they just enjoy the excitement and what the city offers.

This population shift is bringing problems as well as some advantages. Countless poor people must leave their flats in the city because the owners want to sell the buildings or make flats for sale instead of for rent（租金）. In the 1950s, many poor people did not have enough money to move to the suburbs; now many of these same people do not have enough money to stay in the cities.

Only a few years ago, people thought that the older American cities were dying. Some city residents now see a bright, new future. Others see only problems. One thing is sure: many dying cities are alive again.

1.The main idea of the passage is _________.

2. Why did American city residents want to live in the suburbs after World War Ⅱ?

3. According to the 4th paragraph, a great many poor people in American cities _________.

4.We can know from the passage that _________.

In the past, man did not have to think about the protection of his environment. There were not many people on earth, and natural resources seemed to be unlimited.
Today, things are different. The world has become too crowded. We are using up our natural resources too quickly, and at the same time we are polluting our environment with dangerous chemicals. If we continue like this, human life on earth will not survive (继续生存).
Everyone realizes today that if too many fish are taken from the sea, there will soon be none left. Yet, with modern fishing methods, more and more fish are caught. We know that if too many trees are cut down, forests will disappear and nothing will grow on the land. Yet, we continue to use bigger and more powerful machines to cut down more and more trees.
We realize that if rivers are polluted with waste products from factories, we will die. However, in most countries waste products are still put into rivers or into the sea, and there are few laws to stop this.
We know, too, that if the population of the world continues to rise at the present rate, in a few years, there will not be enough food.
What can we do to solve these problems?
If we eat more vegetables and less meat, we will easily get more food. Land that is used to grow crops can feed five times more people than land where animals are kept.
The world population will not rise so quickly if people use modern methods of birth control.
Finally, if we educate people to think about the problems, we shall have a better and cleaner living place in the future.
【小题1】. If we use up our natural resources and go on              our environment, human beings will die out.
【小题2】 If we catch too many fishes from the sea,               will be left.
【小题3】. The world is becoming too                 , so we should use modern methods of
birth control to reduce the population.
【小题4】. Educate people to think about the problems so that the earth will be better and               .
【小题5】. The passage mainly tells us the importance of protecting the environment and
to solve the environmental problems.