资料题：What does the passage mainly discuss（　）。

American cities are similar to other cities around the world.In every country, cities reflect the values of the culture.Cities contain the very best aspects of a society: opportunities for education, employment, and entertainment.They also contain the very worst parts of a society: violent crime, racial conflict, and poverty.American cities are changing, just as American society.After World War I, the population of most large American cities decreased; however, the population in many Sum Belt cities increased.Los Angeles and Houston are cities where population increased.These population shifts to and from the city reflect the changing values of American society.During this time, in the late 1940s and early 1950s, city residents became wealthier, more prosperous.They had more children.They needed more apace.They moved out of their apartment in the city to buy their own homes.They bought houses in the suburbs, areas near a city where people live.These-are areas without many offices or factories.During the 1950s the American"dream" was to have a house in the suburbs.Now things are changing.The children of the people who left the cities in the 1950s are now adults.They, unlike their parents, want to live in the cities.Some continue to move to cities in the Sun Belt.Cities are expanding and the population is increasing in such states as Texas, Florida, and California.Others are moving to more established cities of the Northeast and Midwest, such as Boston, Baltimore and Chicago.Many young professionals, doctors, lawyers, and executives are moving back into the city.They prefer the city to the suburbs because their jobs are there; they are afraid of the fuel shortage or they just enjoy the excitement and opportunities which the city offers.A new class is moving into the cities- -a wealthier, more mobile class.