问题
单项选择题
生态系统的营养级一般不超过五个的原因是（）
A.能量流动的特点是单向逐级递减的，传递效率为10％－20％
B.位于第五营养级的动物是凶猛的，不可能被其它动物所食
C.能量在传递过程中，沿食物链散开，不集中
D.位于第四、五营养级的动物食性不专一
答案
参考答案：A
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生态系统的营养级一般不超过五个的原因是（）
A.能量流动的特点是单向逐级递减的，传递效率为10％－20％
B.位于第五营养级的动物是凶猛的，不可能被其它动物所食
C.能量在传递过程中，沿食物链散开，不集中
D.位于第四、五营养级的动物食性不专一
参考答案：A
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 by industry．If the rapid increase of world population continues at the present rate, there may be much greater to human beings． Some scientists speak of the increase in numbers of people as "population pollution''． About 2,000 years ago, the world population was about 250 million．It a billion in 1850．By 1930 the population was two billion．It is now six billon．It is to double by the year 2020．If the population continues to grow at the same rate, there will be 25 billion people in the a hundred years from now． Man has been the earth's resources more and more over the past years．Some of them are almost gone．Now many people believe that man's greatest problem is how to control the of the population．The material supplies in the world will be far from enough to the human population if the present rate of increase continues． There is already over-crowding in many cities and starvation in some countries．Many people believe that human survival in the future on the answer to the question．
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