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  3. The eight chipmunk species of the Sierra Nevada represent but a few of the 15 species found in western North America, yet the wh

The eight chipmunk species of the Sierra Nevada represent but a few of the 15 species found in western North America, yet the wh

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问题     The eight chipmunk species of the Sierra Nevada represent but a few of the 15 species found in western North America, yet the whole of eastern North America makes do with but one species: the Eastern chipmunk. Why are there so many very similar chipmunks in the West? The presence of tall mountains interspersed with vast areas of arid desert and grassland makes the West ecologically far different from the East. The West affords much more opportunity for chipmunk populations to become geographically isolated from one another, a condition of species formation. Also, there are more extremes in western habitats. In the Sierra Nevada, high elevations are close to low elevations, at least in terms of mileage, but ecologically they are very different.
In paragraph 2, the author indicates that a large variety of chipmunk species exist in western North America because of
选项 A、a large migration of chipmunks from eastern North America in an earlier period
B、the inability of chipmunks to adapt to the high mountainous regions of eastern North America
C、the ecological variety and extremes of the West that caused chipmunks to become geographically isolated
D、the absence of large human populations that discouraged species formation among chipmunks in the East
答案C
解析
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