填空题：烧煤气熔窑内气体流动的动力来源于（）和（）提供的压力，以及（）提供的压力。

题型：阅读理解

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.

小题1:The main idea of the passage is _________.

A．cities around the world

B．the dying cities

C．the changing American cities

D．the changing of the American culture小题2: Why did American city residents want to live in the suburbs after World War Ⅱ?

A．Because older American cities were dying.

B．Because they were richer and needed more space.

C．Because cities included the worst parts of society.

D．Because they could hardly afford to live in the city.小题3: According to the 4th paragraph, a great many poor people in American cities _________.

A．are faced with housing problems

B．are made to move to the suburbs

C．want to sell their buildings