单选题
Why does the author mention the Sierra Nevada in line 10?
A、
To argue that a mom direst route to the West could have boon taken,
B、
To identify a historically significant mountain range in the Went,
C、
To point out the legation of n serious train accident,
D、
To give an example of an obstacle faced by the central pacific.
【正确答案】
D
【答案解析】
根据the Sierra Nevada所在句可知作者提到它是因为它是修建横跨东西铁路的一个巨大的障碍，故D为正确答案。
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