题目内容：
Why would an animal kill itself? It seems a strange question, and yet it is one that has ____26____ some people for a long time. The lemming (旅鼠) is one such animal. Lemmings periodically commit mass____27____, and no one knows just why!The small____28____, which inhabit the Scandinavian mountains, sustain themselves on a diet of roots and live in nests they make underground. When their food supply is ____29____large, the lemmings live a normal, undisturbed life.
However, when the lemmings' food supply becomes too low to support the population, a singular____30____ 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, z journey that may last weeks. The lemmings eat everything in their path, continuing their ____31____ 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 ____32____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____33____ water. In their cross-country journey, the animals must traverse many smaller bodies of water, such as rivers and small lakes. They may ____34____ that the sea is just another such swimmable ____35____. But no final answer has been found to the mystery.
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