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  3. Early naturalists believed two species of beaver lived in North America: dam beavers and bank beavers. The bank species was thou

Early naturalists believed two species of beaver lived in North America: dam beavers and bank beavers. The bank species was thou

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问题     Early naturalists believed two species of beaver lived in North America: dam beavers and bank beavers. The bank species was thought to resemble the muskrat in behavior, living in burrows or lodges and unable to build dams. In fact, dams are primarily a strategy for dealing with annual variations in water levels. If water levels fall in summer, as they do in most of North America, then beavers lodge entrances may be exposed. With stabilized water levels, their homes are much safer. Along deep rivers, where bank beavers are found, this problem seldom arises. But these beavers do know how to build dams, and do so if the need arises, as may occur if they are forced to relocate after felling and consuming all nearby trees.
The passage provides support for which of the following statements about beaver dams?
选项 A、One important function of these dams is to protect beavers homes.
B、Most are built prior to burrow construction.
C、They are found mostly along deep rivers.
D、They are routinely abandoned as nearby forests are depleted.
E、They mainly protect beavers from rising water levels.
答案A
解析 根据三四五可知，水坝的作用是保护海狸的家，选项A正确。B项文中看不出水坝和洞穴的时间顺序。C项根据第三句，如果沿着深河，那么就不需要水坝。D项根据第四句，森林耗尽之后反而需要水坝。E项根据第五句，水位下降的时候才需要水坝。
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