单选题 .  Although the United States cherishes the tradition that 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 had 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 constituted one of the greatest migrations of modern times; quiet 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 (人口调查) 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 populations 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 600-mile 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.1.  A traditional American belief is that ______.