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  3. Here in the United States, before agricultural activities destroyed the natural balance, there were great migrations of Rocky Mo

Here in the United States, before agricultural activities destroyed the natural balance, there were great migrations of Rocky Mo

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问题     Here in the United States, before agricultural activities destroyed the natural balance, there were great migrations of Rocky Mountain locusts (Melanoplus spretus). Great migrating hordes of these insects once darkened the skies on the plains east of the Rockies where crops were often destroyed; the worst years were those from 1874 to 1877. One of these migrating swarms was estimated to contain 124 billion locusts. During another migration in Nebraska it was estimated that the swarm of locusts averaged half a mile high and was 100 miles wide and 300 miles long. Usually, these swarms take off from the ground against the wind, but, once airborne, they turn and fly with it. Warm convection (对流) currents help to lift them, often to great heights. During the great locust plagues the situation in Nebraska became so serious that the original state constitution had to be rewritten to take care of the economic problems. The new document was known as "The Grasshopper Constitution". It is now believed that these locusts were a migratory form or phase of the lesser migratory locust, which is still common there. In this respect, the North American migratory locusts resemble their African relatives. In both regions the migratory forms arise as a result of crowding and climatic factors. Migratory forms are apparently natural adaptations which bring about dispersal when locust populations become too crowded.
    Fortunately for our farmers, the migratory form—the so called spretus species—no longer seems to occur regularly, although there was a serious outbreak as late as 1938 in midwestern United States and Canada. Actually, there is no reason why the destructive migratory form might not again appear if circumstances should become favorable.
Which of the following is NOT mentioned in the passage as influencing the migration of locusts?
选项 A、Darkness.
B、Agricultural activities.
C、Warm air currents.
D、Overcrowding.
答案A
解析 本题可以通过排除法找到答案。文中第一句说：“在美国农业活动破坏生态平衡之前，存在着落基山蝗虫的大量迁徙的现象。”由此可见，B“农业活动”作为影响蝗虫迁徙的因素之一文中已提及，故可排除。文中倒数第四句说：“在这两个地区，迁徙蝗虫的出现是由于拥挤和气候这两个因素造成的。”D在这里已提及，可排除。文中第六句说：“温暖的对流帮助它们上升，时常使它们上升到很高的高度。”由此C可排除。文中唯一没有提到的影响蝗虫迁徙的因素是黑暗。故A正确。
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