单项选择题

Many of the world’s pollution problems have been caused by the crowding of large groups of people into the cities. Supply for the needs of the people leads to further pollution by industry. If the rapid increase in human population continues at the present rate, there may be much greater harm. Some scientists speak of the increase in numbers of people as "population pollution".
About 2,000 years ago, the world population was probably 250 million. It reached a billion in 1850. By 1930, the population was two billion. It was three and a half billion in 1988. It is expected to double by the year 2000. If the population continues to grow at the same rate, there would be 25 billion people in the world a hundred years from now!
Man has been using the earth’s resources more and more rapidly over the years. Some of them are almost gone. Now many people believe that man’s greatest problem is how to control population, in time to come, if the present rate of increase continues. Already there is overcrowding in the cities and hunger in some countries. Can man’s rate of increase continue Many people believe that human survival in the future depends on the answer to this question.

The most serious problem man is facing is thought to be ()

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