(单选题)
The first “it” in Paragraph 3 refers to（指的是）______.
A.being physical fit
B.climbing Everest
C.using oxygen when climbing
D.climbing without oxygen
参考答案：C
参考解析：
细节理解题。根据第三段第一句“The common practice was to use oxygen when climbing, as it, in fact, lowers the altitude（海拔）of the mountain by several thousand feet.”常见的做法是攀登时使用氧气瓶，因为它，事实上降低了高度(海拔)山的几千英尺。可知it指代的是氧气瓶。故选C。
知识点：
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