Mountain climbing be very tiring, and many people feel sick the atmosphere gets thinner.
A. may; while
B. can; as
C. must; though
D. should; for
网友回答
A. may; while
(答案→)B. can; as
C. must; though
D. should; for
解析：考查情态动词和连词，第一空填could，表示客观可能，第二空填as，引导时间状语从句，表示当…时候，随着，句意：爬山可能让人疲劳，随着空气变得稀薄很多人会感到不适。选B。