(A) I can’t complain about this one. “Carbon dioxide gas” is a singular, non-countable noun, so “less” works fine as a modifier. And this part is a little bit tricky, but the second part of the sentence refers to several different “gasses” – a countable, plural noun – so “fewer” is appropriate.

The phrase beginning with “that” (“that have been implicated…”) correctly modifies “other gasses.” It’s also completely fine to use the “-ing” form of the verb after “as well as.”

So we’ll keep (A).


(B) It’s a funny quirk of English: if “as well as” were changed to “and” then we’d want the verbs “burn” and “emit” to be in parallel form. But since we have “as well as”, we’re better off using “emitting”, as we did in (A).

More importantly, I can’t make any sense of the “having been” at the end of the underlined portion. For that reason, we can scrap (B).


(C) You could argue that the pronoun “they” is ambiguous here, since it could refer to “diesel engines” or “gasoline engines”, but I’m not convinced: since “they” is the subject of the second clause, it can generally refer back to the subject of the first clause on the GMAT without causing any trouble.

But we definitely have a modifier problem here: “fewer carbon dioxide” doesn’t make any sense, since “carbon dioxide” is a non-countable noun, and “fewer” can only be used with countable nouns. (If you’re not clear about that concept, try counting the noun out loud: “one carbon dioxide, two carbon dioxides, three carbon dioxides…” That makes no sense at all, right? So “carbon dioxide” is non-countable.)


(D) The first part of the underlined portion isn’t necessarily WRONG, but it definitely isn’t great: “gasoline engines that have a comparable size” is a really crappy way to say “gasoline engines of comparable size.” I just don’t think it makes sense to use the word “have” in this context, since gasoline engines don’t really possess size.

More importantly, we have a problem with the non-countable modifier “less”, since it seems to be modifying both “carbon dioxide gas” (non-countable) and “other gasses” (countable) – and “less” can’t modify a countable noun.


(E) The first part of the underlined portion suffers from the same problem as (D): “that have a comparable size” is a lousy way to say “of comparable size.” But again, I wouldn’t necessarily eliminate (E) based on that issue by itself.

The other problem is the same as in (D): “less” seems to modify “other gasses”, and that doesn’t work. Plus, I’m really not sold on the idea of using “having” to modify “gasses” at the end of the underlined portion – the version in (A) (“gasses that have…”) seems a little bit better.