This development-and its strong implications for US politics and economy in years ahead-has enthroned the South as America's most densely populated region for the first time in the history of the nation's head counting.
Altogether, the US population rose in the 1970s by 23.2 million people-numerically the third largest growth ever recorded in a single decade. Even so, that gain adds up to only 11.4 percent, lowest in American annual records except for the Depression years.
Americans have been migrating south and west in larger number since World War II, and the pattern still prevails.
Three sun belt states—Florida, Texas and California—together had nearly 10 million more people in 1980 than a decade earlier. Among large cities, San Diego moved from 14th to 8th and San Antonio from 15th to 10th-with Cleveland and Washington D.C. dropping out of the top 10.
Not all that shift can be attributed to the movement out of the snow belt, census officials say, "Nonstop waves of immigrants played a role, too—and so did bigger crops of babies as yesterday's 'baby boom' generation reached its child bearing years."
Moreover, demographers see the continuing shift south and west as joined by a related but newer phenomenon: More and more, Americans apparently are looking not just for places with more jobs but with fewer people, too.
Regionally, the Rocky Mountain States reported the most rapid growth rate—37.1 percent since 1970 in a vast area with only 5 percent of the US population.
Among states, Nevada and Arizona grew fastest of all: 63.5 and 53.1 percent respectively. Except for Florida and Texas, the top 10 in rate of growth is composed of Western states with 7.5 million people—about 9 per square mile.
The flight from over crowdedness affects the migration from Snow Belt to more bearable climates.
Nowhere do 1950 census statistics dramatize more the American search for spacious living than in the Far West. There, California added 3.7 million to its population in the 1970s, more than any other state.
In that decade, however, large numbers also migrated from California, mostly to other parts of the West. Often they chose—and still are choosing—somewhat colder climates such as Oregon, Idaho and Alaska in order to escape smog, crime and other plagues of urbanization in the Golden State.
As a result, California's growth rate dropped during the 1970s, to 18.5 percent—little more than two-thirds the 1960S growth figure and considerably below that of other Western states.
Discerned from the perplexing picture of population growth the 1980 census provided, America in 1970s ____.

A. enjoyed the lowest net growth of population in history
B. witnessed a southwestern shift of population
C. underwent an unparalleled period of population growth
D. brought to a standstill its pattern of migration since World War Ⅱ