单项选择题

TEXT D
Going Forth, The Nations Multiply Unevenly
Despite was, famines, and epidemics, Earth’s population is booming ahead to new records—with no end insight.
Every day, the world adds enough people to populate a medium-sized city in the US. In one month, the number of new world citizens equals the population of New York City. Every year, there are 90 million more mouths to feed, more than the total population of Germany.
Several factors are propelling this rapid growth, including an element that is often overlooked the huge number of teenagers who are becoming mothers, particularly in the countries of sub-Saharan Africa In four African nations m Niger, Mali, Sierra Leone, and Ivory Coast—1 out of every 5 adolescent females of childbearing age has a baby annually. The US Bureau of the Census says this high rate of motherhood among teens has helped to maintain the high pace of births across most of the African continent By starting a family early, a typical woman is Somalia, for instance, has seven children during her lifetime. Equally large families are the rule in Zambia, Zaire, Uganda, Mauritania, Mall, Malawi, and Ethiopia.
The current record-holder for fertility is strife-torn Rwanda, where a typical mother has at least eight or nine children. While population experts often focus on Africa’s problems, analysts note that teenaged mothers are also far more prevalent in the United States than in France, Germany, Italy, Sweden, or Britain.
This issue— "babies having babies" —has recently gained prominence in the US. Teenaged motherhood in the US has fueled an expansion of the state federal welfare system and brought cries for welfare reform from lawmakers.
With its high rate of teen births, the US now ranks alongside Indonesia and parts of South America and only modestly ahead of Mexico, India, and Pakistan.
Overall, the fertility rate among Americans remains relatively low at 2.1 births per woman—about the replacement level. Although the US population is expected to climb steadily, from 260 million today to 323 million by 2020, most of that growth will come from immigration
The Census Bureau estimates that in Haiti, where thousands of citizens are trying to flee to the US because of military oppression and poverty, AIDS will cut the annual growth rate during the next 25 years from 2.1 percent to 1.3 percent.
The decline in growth is even slim-per in the Central African Republic, where rates will dip from 2.4 percent to 0.7 percent. In Thailand, which already had low birth rates AIDS will drive population downward to 0.8 percent a year. In the 16 countries that are hit hardest, AIDS will lower populations by 121 million over expected projections by 2020. In Africa, the impact of AIDS is so great that trends toward longer life spans during the past 40 years are being reversed Some nations will suffer declines in average life spans of 10 to 30 years compared with expected life spans without AIDS.
In the US, where AIDS is also a substantial problem, the impact will be lower because the disease is mostly limited to homosexuals and drug users, says Peter Way, a Census Bureau researcher. In many African nations, AIDS is prevalent among the heterosexual population, which sharply boosts infant mortality.
A compelling chapter in the research deals with aging. Today the median age in developed countries is 35, and in developing nations me only 23. By 2020, the corresponding figures will be 42 and 28. Today there are fewer adults over 60 ($25 million), than children under 5 (636 million). As the world population ages, by 2020 the number over 60 will be more than; 1 billion, while those under 5 will total 717 million.
Sub-Saharan African countries______.