This is a gnarly question that operates at the intersection of parallelism and meaning. The basics of parallelism aren’t all that hard, but then the GMAT turns them into a goofy logic puzzle. More on that in our
webinar on parallelism and meaning if you haven’t already seen it.
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(A) changing the measured strain across a fault zone, and varying
Superficially, the parallelism doesn’t look awful here, but it’s not quite right. The sentence revolves around a list of “warning signs for a major earthquake”, and the non-underlined portion features the first two items in the list (“sudden fluctuations” and “tilting and other deformations”), both of which are nouns.
So we need those last two items in the list to be nouns that are logically parallel to “sudden fluctuations” and “tilting and other deformations.” But I don’t think we actually have nouns in the underlined portion: “changing” and “varying” seem to be structured as modifiers, not nouns.
To be fair, this is tricky as hell: as discussed in our
article on “-ing” words, “-ing” words could be modifiers or nouns (also known as gerunds), depending on how they’re used. In this particular case, there’s no clear indication that “changing” and “varying” are nouns. If (A) said “the changing of measured strain” and “the varying of electrical properties”, then you could convince me that these are parallel nouns.
Another way of thinking about this: what, exactly, are the signs of major earthquakes? Is it “changing the measured strain across the fault zone”? That doesn’t really make sense: the phrase suggests that the act of
changing the measurement is the “sign of a major earthquake.” In reality, the sign of the earthquake is
changes in the strain – not changes to the measurement itself.
So whether you’re thinking about meaning or structure, (A) is wrong. If you wanted to be conservative on your first pass through (
never a bad idea!), you could keep (A), but there are better options available.
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(B) changing measurements of the strain across a fault zone, and varying
(B) is a slightly better version of (A), but it still isn’t quite good enough. Sure, “changing measurements” is clearly a noun now, with “changing” serving as an adjective. But it still incorrectly implies that the measurements are changing, and not the strain itself. And “varying” still looks like a modifier.
So (B) is out.
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(C) changing the strain as measured across a fault zone, and variations of
(C) is more tempting than (A) or (B), but it’s still not quite right. “Changing the strain” still has the same problem we described in (A): “changing” isn’t necessarily a noun here, and the act of “changing the strain” really isn’t a sign of a major earthquake – it’s much clearer to say that “changes in the strain” are a sign of an earthquake.
So we can probably do better than (C), too.
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(D) changes in the measured strain across a fault zone, and variations in
Here we go! Now the list consists only of nice, straightforward, unambiguous nouns: “warning signs for a major earthquake include… changes in the measured strain… and variations in the electrical properties of rocks.” Those are nouns, without any doubt.
And just as importantly, they make perfect sense as warning signs for an earthquake. Let’s keep (D).
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(E) changes in measurements of the strain across a fault zone, and variations among
The differences between (D) and (E) are pretty subtle. (E) gives us a pair of nouns, so it’s structurally parallel – but the meaning doesn’t work anymore. “…changes in
measurements of the strain” are NOT signs of an earthquake: if the measurement changes, that doesn’t necessarily mean that the strain itself has changed. For that reason alone, we could ditch (E).
Plus, the phrase “variations among” doesn’t quite work. If we talk about “variations among curry recipes”, we’re saying that different recipes have different characteristics. So “variations among electrical properties of rocks” would suggest that the different electric properties of rocks are… well, different from each other. But that isn’t what we’re trying to say: the point of the sentence is that the electrical properties of individual rocks vary before a major earthquake. So “variations in” is much, much clearer.
Ugh. Having fun yet? (E) is out, and (D) wins.
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