Here's the
official explanation provided by the GMAC for this question:
The original version can be read as illogically indicating that Wilson’s withdrawal and periods of lethargy comprised the entire times in which Wilson was alert. In principle, one could be alert and at the same time lethargic or withdrawn (from other people). However, the earlier part of the sentence says that he had outbursts of temper and tears. These would necessarily have been during times when he was alert and neither lethargic nor withdrawn. Thus, the original wording is contradictory. The phrase
periods that were lethargic is indirect and illogical, appearing to claim that the periods, rather than the person, were lethargic.
Option A: This can be read as illogically claiming that Wilson’s alert times consisted of alternating periods of lethargy and withdrawal. The phrase
periods that were lethargic is indirect and illogical, appearing to claim that the periods, rather than the person, were lethargic.
Option B: Correct. This logically, directly, and concisely expresses the relationship between the periods of alertness and the periods with which those are contrasted. The phrase
alternated with clearly indicates that the periods of alertness were not themselves periods of lethargy and withdrawal.
Option C: This illogically treats
alternated as a transitive verb; it says that periods of lethargy and periods of withdrawal were things that the periods of alertness alternated, as if the periods of alertness acted to bring about those effects. The phrases
that were of lethargy and [that were]
of withdrawal are unidiomatic, awkward, verbose, and jarringly indirect.
Option D: This illogically treats
alternated as a transitive verb; it says that withdrawal and periods of lethargy were things that the periods of alertness alternated, as if the periods of alertness acted to bring about those effects.
Option E: This illogically claims that Wilson’s alert times consisted of alternating periods during each of which he was both lethargic and withdrawn.
The correct answer is B.
Please note that I'm not the author of this explanation. I'm just posting it here since I believe it can help the community.