Here's the
official explanation provided by the GMAC for this question:
The argument reported in this sentence is based on an analogy between two practices: the established practice of giving polio vaccine to people in general and the proposed practice of getting people in general to eat healthful diets. The doctor argues that because the proposed practice would fulfill a need similar to that fulfilled by the established practice, the proposed one should also become established. Parallel wording is needed to draw an effective analogy between the two. The phrasing should make it clear what is being compared to what, and in what respect.
Option A: Correct. This effectively expresses the analogy between the established practice and the proposed practice. It does so with relevantly parallel verb phrases,
polio vaccine is given … and
mass dietary change is needed.
Option B: With this structure, the sentence illogically indicates that polio vaccine is like mass dietary change in that both are needed to protect certain people from the effects of their eating habits.
Option C: Without a comma after
vaccine, the ensuing phrase
which is given appears to modify
polio vaccine restrictively, delineating the subset of polio vaccine that is
given to every person. The intended function of
similar is unclear. It could mean that mass dietary change is similar in some unspecified way to this subset of polio vaccine (or to polio vaccine in general) and that, furthermore, mass dietary change is needed for the mentioned purpose. Or perhaps it is mistakenly used as a synonym for
similarly. On that interpretation, the sentence illogically indicates that polio vaccine is like mass dietary change in that both are needed to protect certain people from the effects of their eating habits.
Option D: In this usage,
while is ambiguous. It could mean
although or
during the time when. The commas around the phrase
to protect the few who might actually contract polio make it function as a parenthetical bit of information about polio vaccine, so the core claim conveyed by this wording is equivalent to either (1)
although polio vaccine is given to every person, mass dietary change is needed to protect the significant number … or (2)
during the time when polio vaccine is given to every person, mass dietary change is needed to protect the significant number. Either way, the sentence is not entirely coherent.
Option E: This incoherently and ungrammatically draws a comparison between a noun phrase (
the giving) and a clause (
mass dietary change is needed … ). The placement of
actually at the end of the phrase rather than adjacent to the verb
contract is awkward and stilted; it would be better omitted.
The correct answer is A.
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.