单项选择题

According to the best historical and archaeological evidence, it is estimated that it took about 800 thousand to 1 million years for the earth’s population to reach the 250 total which existed at the end of the first century ’after Christ. For some time after that, disease, famine, and war kept the population increase down to a fraction of 1 percent a year so that more than 15 centuries passed before the population reached 500 million. But in the next 250 years, up to 1850, the population of the world shot up to the 1 billion mark, and today it has reached 3 billion. It is predicted by United Nations investigators that in the next 35 years the population of the world will double.
When experts are asked what the most effective measure which overpopulation poses, they reply that at least three measures can be considered:
(1) increasing sources and supply of food for underdeveloped countries; (2) increasing the industrialization of underdeveloped countries; (3) regulating conception and births. The experts add, however, that none of the proposed measures Can be effective by itself, that all must be combined into an integrated program. They also agree that without some regulation of conceptions and births, any other measures are doomed to failure.

It is estimated that in the year（）

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