问题详情

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

请帮忙给出正确答案和分析，谢谢！