阅读选择：阅读下面短文，请从短文后所给各题的四个选项(A、B、C、D)中选出一个最佳选项。 Basic to any understanding of Canada in the 20 years after the Second World War is the country's impressive population growth. For every three Canadians in 1945, there were over five in 1966. In September 1966 Canada's population passed the 20 million mark. Most of this surging growth came from natural increase. The depression of the 1930s and the war had held back marriages, and the catching-up process began after 1945. The babyboom（出生高峰期）continued through the decade of the 1950s, producing a population increase of nearly fifteen percent in the five years from 1951 to 1956. This rate of increase had been exceeded only once before in Canada's history, in the decade before 1911, when the prairies were being settled. Undoubtedly, the good economic conditions of the 1950s supported a growth in the population but the expansion was also the result of earlier marriages and an increase in the average size of families. In 1957 the Canadian birth rate stood at 28 per thousand, one of the highest in the world. After the peak year of 1957, the birth rate in Canada began to decline. It continued falling until 1966. Partly this decline reflected changes in Canadian society. Young people were staying at school longer; more women were working; young married couples were buying automobiles or houses before starting families; and rising living standards were cutting down the size of families. It appeared that Canada was once more falling into step with the trend toward smaller families that had occurred all through the Western world since the time of the Industrial Revolution. Although the growth in Canada's population had slowed down by 1966, another large population wave was coming over the horizon. It would be composed of the children of the children who were born during the period of the high birth rate prior to 1957.