When I try to understand ______ that prevents so many Americans from being as happy as one might expect, it seems to me that there are two causes.

A．why it does

B．what it does

C．what it is

D．why it is

旅游地理学研究的最终目的是（）

A.研究人类旅游与地理环境之间关系

B.研究人类旅游与社会经济之间的关系

C.促进旅游业与社会经济的发展

D.促进地理环境的发展