TPO36READING chipmunk

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.这句话该怎么理解？

关于这篇阅读的第三题：In paragraph 2, the author indicates that a large variety of chipmunk species exist in western North
America because of
(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
为什么答案还要选B，我个人觉得C是正确的啊

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

明显C，答案错了