单项选择题

Passage One
Just 30 years ago some 700 million people lived in cities. Today the number stands at 1,800 million, and by the end of the century it will be up to 3,000 million-more than half the world’s estimated population. By the year 2000 an estimated 650 million people will crowd into 60 cities of five million or more-three quarters of them in the developing world. Only a single First World city-metropolitan Tokyo, which will have 24 million people-is expected to be among the global top five; London, ranked second in 1950 with ten million people, will not even make 2000’s top 25. In places where rates of natural population increase exceed three percent annually-meaning much of the Third World-that alone is enough to double a city’s population within 20 years. But equally powerful are the streams of hopeful migrants from the countryside.
What faces and confuses urban planners is the huge scale of these trends. There have never been cities of 30 million people, let alone ones dependent on roads, sewer and water supplies barely adequate for urban areas a tenth that size. And the flood of new arrivals in swelling Third World cities far overtakes the supply of jobs-particularly as modem industries put a premium on technology rather than manpower. So it will be virtually impossible to find permanent employment for 30 to 40 percent of the 1,000 million new city inhabitants expected by the year 2000.
Despite the terrible conditions that the city newcomers face, their numbers are growing at rates as much as twice that of the cities themselves-and every step taken to improve their living conditions in the slums only attracts more migrants.
Which of the following is the main topic of this passage