Mountain climbing must be very tiring, ,you might feel sick with the air getting thinner.
A. In addition
B. In turn
C. In return
D. In case
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(答案→)A. In addition
B. In turn
C. In return
D. In case
解析：考查短语辨析。In addition 此外，In turn 轮流，In return 作为回报，In case 以免，句意：爬山一定非常累，此外随着空气越来越稀薄你可能会感到不舒服。所以选A。