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  3. Why would an animal kill itself? It seems a strange question, and yet it is

Why would an animal kill itself? It seems a strange question, and yet it is

游客2023-08-21  18
问题     Why would an animal kill itself? It seems a strange question, and yet it is one that has【B1】______some people for a long time. The lemming (旅鼠) is one such animal. Lemmings periodically commit mass【B2】______, and no one knows just why!
    The small【B3】______, which inhabit the Scandinavian mountains, sustain themselves on a diet of roots and live in nests they make underground. When their food supply is【B4】______large, the lemmings live a normal, undisturbed life.
    However, when the lemmings’ food supply becomes too low to support the population, a singular【B5】______commences. The lemmings leave their nests all together at the same time, forming huge crowds. Great numbers of the lemmings begin a long and hard journey across the Scandinavian plains, a journey that may last weeks. The lemmings eat everything in their path, continuing their【B6】______march until they reach the sea.
    The reason for what follows remains a mystery for zoologists and naturalists. Upon reaching the coast, the lemmings do not stop but swim by the thousands into the surf. Most【B7】______only a short time before they tire, sink, and drown.
    A common theory for this unusual phenomenon is that the lemmings do not realize that the ocean is such【B8】______water. In their cross-country journey, the animals must traverse many smaller bodies of water, such as rivers and small lakes. They may【B9】______that the sea is just another such swimmable【B10】______. But no final answer has been found to the mystery. [br] 【B10】
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答案 obstacle
解析 此空的前面为another such swimmable，说明此空需要填入一个名词，而且是单数名词。obstacle意为“障碍(物)”。
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