单项选择题

A new report of the United Nations shows that, if the present growth rate of 2 percent per year continues, today’s world population of 5.1 billion will hit 6.4 billion by the year 2000.
What’s more, the great part of the growth—9 of every 10 people added to the earth’s population—will be in the poor and undeveloped countries. These are the nations where providing enough food for billions of people already is proving to be a headache problem.
By the year 2000, today’s "have not" nations will have a total population of 5 billion people, nearly four fifths of the world’s population.
Food isn’t the only problem that such a population explosion presents. The more people there are and the worse their living conditions, the greater grow the possibility of all kinds of social problems.
In 1830, world population reached 1 billion. It took only 100 more years to add another billion to world population; just 30 more to add a third billion. And it took just 15 more years to reach the 4 billion mark in 1975.
Actually, the world’s birth rate is falling. But so is death rate, as medical advances have made it possible for man to live longer than before. Such advances have also reduced baby death rate. Unless population growth is reduced, the world population may reach 12 billion in a century. Is the earth capable of providing a good life for so large a population
 

（）, the world may have population of 12 billion in a century.

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