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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.
The passage supports which of the following conclusions?
选项 A、The outbreak of locusts in 1938 was more serious than any other in history.
B、Nebraska farmers had no locust problems in the years 1874—1877.
C、There is a possibility that crops in the Unite States might be destroyed by locusts in the future.
D、There is a chance that African migratory locusts may make their way to the United States.
答案C
解析 本题为推断题。文中最后一句说：“事实上，如果环境变得对蝗虫有利，没有理由说，毁灭性的蝗虫迁徙不会再出现。”由此我们可以推断出这样一种可能：美国的农作物将来还会受到蝗虫的破坏。所以正确答案为C。本题亦可通过排除法找到答案。文中倒数第二句中说：“虽然在1938年美国的中西部和加拿大有一次严重的蝗灾爆发。”可见，文中并未说这次爆发的蝗灾比历史上任何一次都严重，故A可排除。文中第二句说“蝗灾造成最严重危害的时期是1874年至1877年”，但并没有提此期间内布拉斯加州的农民没有蝗灾问题，由此B可排除。文中第十句说：“在这一点上，北美的迁徙蝗虫与它们的非洲同类有相似之处。”文中并未提及或暗示非洲的迁徙蝗虫会迁徙到美国，故选项D可排除。
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