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  3. The cause of the decline of North Africa is popularly attributed to climatic changes, the theory being that the area became hott

The cause of the decline of North Africa is popularly attributed to climatic changes, the theory being that the area became hott

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问题     The cause of the decline of North Africa is popularly attributed to climatic changes, the theory being that the area became hotter and drier and the people were forced to abandon a thriving civilization. However, some geologists, after carefully weighing the evidence, have challenged the conclusion that the climate has changed in any important way since Roman times. Although climatological factors may also have contributed, it was man who destroyed the balance of nature responsible for maintaining soil fertility and moisture.
    Why is it that under certain circumstances,  the plant cover is unable to regenerate itself, thus setting the stage for serious erosion (腐蚀)? To answer this question, let us take an extreme example, the destruction of tropical rain forests. In general, the equatorial (赤道附近的) soil is poor: forests can exist in these regions only because they are part of a balanced cycle. All that the forest produces is returned to the forest. The organic matter that falls from the trees constitutes the humus (腐殖土) that the forest requires. When people cut down all the trees to make room for crops, the soil is laid bare and, deprived of shade, heats up. An increase in soil temperature increases the rate of decomposition (分解) of organic matter. Organic nitrogen (氮) is converted into soluble ammonia (氨) and nitrates (硝酸盐), which the rains quickly leach (沥滤，溶滤)away.  It has been calculated that a rise in temperature from 77 to 78.8 may increase the loss of nitrogen by fifteen to twenty pounds per acre per year. With the fertility of soil destroyed, an irreversible change from forest to desert can be initiated.
The main idea of this passage is _______.
选项 A、it is nature itself that should take the blame for the decline of North Africa
B、climatic changes coupled with forest abuse result in the decrease of forests in North Africa
C、without sufficient forests, the rains will take away all the organic nitrogen
D、since Roman times the rain forests in North Africa has been decreasing gradually
答案B
解析 主旨题要围绕主题、概括全文主要内容。本文由两段组成，第1段提出论点，第2段给出具体例子进行证明，重点在于第1段的论点。选项B比较全面地概括了本文论点。
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