Although there is great variation in how mountain climbers train, no study has been done to determine why most mountain climbers who train at high altitudes do not increase their lung capacity and, conversely, why so many mountain climbers who never train at high altitudes shall.
The original sentence does not use parallel verb forms in making a comparison. The sentence compares mountain climbers who train at high altitudes and mountain climbers who never train at high altitudes.
Unraveling the sentence by examining this comparison exposes the absence of parallel verb forms; do and shall are not parallel. Choice E changes shall to do and is the correct answer.
Answer - Choice E
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