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    The population problem may be the 1 one of the world today. The world's population is growing 2. Two thousand years ago, there were only 250 million people 3  the earth. Four hundred years ago, the number was4 500 million. But at the beginning of the 5 century, the world's population was about 1,700 million. In 1970, this number was 3,600 million. In 1990, the number was five billion.

    A 6 report said that the world's population 7 six billion by the end of the 20th century. This is just ten 8 after it    9 five billion. The UN estimated(估计)the world's population will grow by more than 10 by the year 2050.