题目： 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, a 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.
