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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.
According to the passage, the worst destruction by locusts in the plains area east of the Rockies occurred during the______.
选项 A、eighteenth century
B、early nineteenth century
C、late nineteenth century
D、twentieth century
答案C
解析 文中第二句说：“这些昆虫的大规模的成群迁徙曾经使落基山脉的东部平原暗无天日，那里的庄稼通常被破坏掉；最糟糕的年份是1874年至1877年间。”C“19世纪晚期”，与文中内容相符。
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