单项选择题

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The American baby boom after the war made the U.S. advice unconvincing to poor countries that they restrain their births. However, there has hardly been a year since 1957 in which birth rates have not fallen in the United States and other rich countries, and in 1976 the fall was especially sharp. Both East Germany and West Germany have fewer births than they have deaths, and the United States is only temporarily able to avoid this condition because the children of the baby boom are now an exceptionally large group of married couples.
It is true that Americans do not typically plan their births to set an example for developing countries. We are more affected by women’s liberation: once women see interesting and well-paid jobs and careers available, they are less willing to provide free labor for child raising. From costing nothing, children suddenly come to seem impossibly expensive. And to the high cost of children are added the uncertainties introduced by divorce, couples are increasingly unwilling to subject their children to the terrible experience of marital breakdown and themselves to the difficulty of raising a child alone.
These circumstances-women working outside the home and the instability of marriage tend to spread with industrial society and they will affect more and more countries during the remainder of this century. Along with them goes social mobility, ambition to rise in the urban world, a main factor in bringing down the births in the nineteenth century.
Food shortage will happen again when the reserves resulting from the good harvest of 1976 and 1977 have been consumed. Urbanization is likely to continue, with the cities of the developing nations struggling under the weight of twice their present populations by the year 2000. the presently rich countries arc approaching a sable population largely because of the changed place women, and they incidentally are setting an example of restraint to the rest of the world. Industrial society will spread to the poor countries, and aspirations will exceed resources. All this will lead to a population in the twenty-first century that is smaller than was feared a few years ago. For those anxious to see world population brought under control, the news is encouraging.
A chief factor in bringing down the births in Europe in the 19^th century is