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题目材料：
In 1995, ecologists started to reintroduce the wolf into Yellowstone Park, where the wolf had been eliminated intentionally half a century earlier. Rangers expected a rise in the number of wolf would place limits on the elk population and on the populations of many mid-size mammals. One unexpected consequence was a dramatic rise in the population of foxes, which in turn helped to control many rodent populations.
以上解析由 考满分老师提供。
