2. 外语
  3. Populations tend to grow at an exponential (指数的) rate. This means that they progressively double. As an example of this type of

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

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问题     Populations tend to grow at an exponential (指数的) 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 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 1979 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 high 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 Colombia, 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.
According to the passage, what helps to explain why the population problem has come on "all of a sudden"?
选项 A、The penny which doubles itself every day for one month.
B、The time span of at least two million years in human history.
C、An illustration of the exponential growth rate given by the author.
D、The large amount of money you would luckily make after the fourth week.
答案C
解析 本题是一道具体细节题。问按照文章内容，什么可以帮助解释人口问题突然出现的原因。文章第一段和第二段的第一句话指出，人口趋向于以指数比率增长。这就意味着它是以累进方式倍增。可以给这种增长率打个比方，如果拿出1美分，以一个月为期，每天加倍，一周以后，仅增为64美分，但是4周以后，便超过100万美元了。这就帮助解释了为什么会突然冒出那么多人口。因此，本题的正确答案应是C。
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