Internal combustion engines show an inherent tradeoff between power an

Magoosh Official Explanation:

This sentence focuses on a unique idiom. This idiom is used for comparisons that describe how two quantities change — both getting bigger together, or both getting smaller, or one bigger & one smaller, etc. The structure of this idiom is

the [comparative adjective][clause #1], the [comparative adjective][clause #2]

This is a full, independent clause on its own, and does not constitute a run-on sentence. Also, the words “the” are crucial to this idiom. It’s a very strange idiom, almost certainly one that will confuse non-native speakers. Fortunately, it always has the same structure, so you can learn how to use it.

In this sentence, what are the two comparative adjectives? The first one is “faster”, a single word, and the second, slightly longer, is “less fuel-efficient.” The general format, therefore, should be

the faster ……, the less fuel-efficient …..

After each comparative, we should have a full [noun] + [verb] clause. Choice (C) is the only one that follows this pattern. All the others — the “if”, the “when”, the “that much less fuel-efficient” — all of these variants are incorrect. This idiom has a very precise structure, and only choice (C) reproduces this structure correctly in both halves of the sentence. Sometimes that’s all you need to find the answer!