Here's the
official explanation provided by the GMAC for this question:
In the original version of this sentence, the opening phrase,
Drug therapy can supplement is followed by an independent clause,
it does not replace psychotherapy ... leaves
supplement with no object. The sentence could be made grammatical and coherent by changing
Drug therapy can supplement to
Drug therapy can supplement psychotherapy. However, there are ways to make the sentence more concise by avoiding the awkward repetition of
psychotherapy as answer choice D does.
Option A: This mistakenly treats
supplements as an intransitive verb, claiming that
drug therapy can supplement but not mentioning anything that it supplements. Thus, it is ungrammatical. The referent of
they is not entirely clear. The intended significance of
as is also unclear; it could mean
while or
because, but neither meaning would be entirely coherent.
Option B: Like answer choice A, this mistakenly treats
supplement as an intransitive verb. Furthermore, the referent of the plural pronoun
they is unclear; it has no plural antecedent within the sentence.
In the treating of a disease is awkward and indirect.
Option C: This structure makes the referent of
they unclear. The intended significance of
as is also unclear; it could mean
while or
because, but neither meaning would be entirely coherent in this context.
Option D: Correct. This is grammatically correct and expresses a coherent meaning. It does so concisely by putting both
supplement and
not replace within the scope of the single auxiliary verb
can.
Option E: Like answer choice A, this mistakenly treats
supplement as an intransitive verb.The intended connection between the two clauses is unclear.
The correct answer is D.
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.
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