单项选择题

TEXT A
Although the United States cherishes the tradition it is a nation of small towns and wide open spaces, only one in every eight Americans now lives on a farm. The recent population trend has been a double one, toward both urbanization and suburbanization. Metropolitan areas have grown explosively in the past decade, and nearly half this increase has been in the suburbs. With the rapid growth of cities has come equally rapid decentralization. The flight of Americans from the central city to the suburbs constitutes one of the greatest migrations of modem times; quite residential sections outside cities have become conglomerations of streets, split-level houses, and shopping centers.
This spurt of suburban expansion, however, does not alter the basic fact that the United States has become one of the most urban nations on the face of the earth. Census Bureau figures show that the rural population has been shrinking steadily since 1830. When the United States became a nation it had no large cities at all; today some fifty cities have population of more than 258,000. Mammoth complexes of cities are developing in the area of the East Coast and the east north-central states, on the Pacific and Gulf coasts, and near the shores of the Great Lakes. Some sociologists now regard the entire 600mile stretch between Boston and Washington D. C. - an area holding a fifth of the country’s population -- as one vast city or, as they call it, megalopolis.
One aspect of the recent population trend is the ______.