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  3. Populations tend to grow at an exponential rate. This means that they progressively double. As an example of this type of growth

Populations tend to grow at an exponential rate. This means that they progressively double. As an example of this type of growth

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问题     Populations tend to grow at an exponential rate. This means that they progressively double. As an example of this type of growth rate, take one penny and double it every day for one month. After the first week, you would have only 64 cents, but after the fourth week you would have over a million dollars.
    This helps explain why the population has come on "all of a sudden". It took from the beginning of human life to the year 1830 for the population of the earth to reach one billion. That represents a time span of at least two million years. Then it took from 1830 to 1930 for world population to reach 2 billion. The next billion was added by 1960, only thirty years, and in 1975 world population reached 4 billion, which is another billion people in only Fifteen years.
    World population is increasing at a rate of 9,000 per hour, 220,000 per day, 80 million per year. This is not only due to higher birth rates, but to lower death rates as well. The number of births has not declined at the same rate as the number of deaths.
    Some countries, such as Columbia, Thailand, Morocco, Costa Rica, and the Philippines, are doubling their populations about every twenty-one years, with a growth rate of 3.3 percent a year or more. The United States is doubling its population about every eighty-seven years, with a rate of 0.8 percent per year. Every time a population doubles, the country involved needs twice as much of everything, including hospitals, schools, resources, food and medicines to care for its people. It is easy to see that this is very difficult to achieve for the more rapidly growing countries.
选项 A、the growth of world population
B、one type of the exponential rate
C、the population problem of more rapidly growing countries
D、the possible ways of dealing with the rapid population growth
答案A
解析 文章主旨题。本文通过一系列的数据，表明了世界人口的增长情况，并介绍了由此带来的问题。显然，主要讨论了世界人口增长问题。
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