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

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

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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(蝗虫). Great migrating hordes of these insects
once darken the skies on the plains east of the Rockies where crops【S1】______
were often destroyed; the worst years were those between 1874 to【S2】______
1877. One of these migrating swarms was estimated to contain 1. 24
billion locusts. During the another migration in Nebraska it was【S3】______
estimated that the swarm of locusts averaged half a mile high and
was 100 miles wide and 300 miles long. Usually, these swarms take
up from the ground against the wind, but, once airborne, they turn【S4】______
and fly with it. Warm convection currents(对流)help to lift them,
often at great heights. During the great locust plagues the situation【S5】______
in Nebraska became so serious that the original state constitution
had to be rewritten to take care the economic problems. The new【S6】______
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, that is still common there. In this respect,【S7】______
the North American migratory locusts resemble their African
relatives. In both regions the migratory forms rise as a result of【S8】______
crowding and climatic factors. Migratory forms are apparently
natural adaptations which bring about dispersal when locust
populations become too crowded. Unfortunately for our farmers, the【S9】______
migratory form 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
migratoiy form might not again appear if circumstances become【S10】______
favorable.
【S1】
选项
答案darken—darkened
解析 时态错误。这里说“蝗虫群曾经遮天蔽日”，有很明显的时态标志词once表示过去，因此应使用一般过去时。
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