Night reader wrote:Synthetic oil burns less efficiently than natural oils do.
A. Synthetic oil burns less efficiently than natural oils do.
B. Synthetic oil burns less efficiently than does natural oil.
C. Synthetic oil burns less efficiently than natural oil.
D. Synthetic oils burn less efficiently than natural oils do.
E. Synthetic oils burn less efficiently than do natural oils.
Okay: The "inversion" NightReader discussed does not matter and can't differentiate between right answers on the GMAT. Repeat:
the "inversion" of word order in the comparison does not matter. Not only on the GMAT, but in any proofreading manual,
(D) and (E) above are exactly the same in terms of correctness.
In fact, the only one that seems wrong above is (A), and even that (singular/plural comparison error) isn't enough to eliminate a choice for certain on the GMAT. The argument against (C) is that it drops the verb in the second part of the sentence, but this is okay. In fact, it is true that we're comparing oil-to-oil in all five choices; the only reason the verb is necessary is that, sometimes, the two compared things can be nonparallel in terms of sentence structure. For example:
"Furnaces burn synthetic oil less efficiently than does natural oil."
The above sentence is clearly wrong -- the reason is that "natural oil" is acting as a verb's
subject, making it parallel to "furnaces," which is an illogical comparison. Since "synthetic oil" is a verb's
object, natural oil needs to be as well:
"Furnaces burn synthetic oil less efficiently than they do natural oil."
Note that another wrong version of the sentence would be
"Furnaces burn synthetic oil less efficiently than natural oil," because it would be ambiguous as to which of the above scenarios is in fact the case.
However, in the original question posted by Night Reader, there's only one noun in each part of the sentence -- oil. Thus it doesn't matter that, in (C), this ambiguity might exist; the second "oil" is automatically parallel to the only other noun in the sentence, "oil," --
each is the subject of the verb "burns," which is implied/ellipsed in the latter part of the sentence.
Thus, (C) is the most concise and still correct, so on test day, I'd choose it. That being said, this would never appear on the GMAT, since 4 of the 5 answers are more or less interchangeable in terms of correctness. I would recommend looking at ALL four examples here as potentially correct options; on the real test, you'd only see one of these four possibilities (although it equally well be any of them!) among the answers, and that would be the correct construction of a comparison.
