The first time I read the original sentence in the below SC problem, I thought to myself: wait, what? What are you actually trying to say? I knew immediately that this would be a good one to discuss with all of you. :)
Let’s try it out (1 minute 15 seconds) and then we’ll dive in. This question is from the free problem set included in the new GMATPrep 2.0 version of the software.
* ” Displays of the aurora borealis, or “northern lights,” can heat the atmosphere over the arctic enough to affect the trajectories of ballistic missiles, induce electric currents that can cause blackouts in some areas and corrosion in north-south pipelines.
“(A) to affect the trajectories of ballistic missiles, induce
“(B) that the trajectories of ballistic missiles are affected, induce
“(C) that it affects the trajectories of ballistic missiles, induces
“(D) that the trajectories of ballistic missiles are affected and induces
“(E) to affect the trajectories of ballistic missiles and induce”
This was my thought process as I read that first sentence:
|Displays of the aurora borealis, or “northern lights,” can heat||Core sentence: displays can heat|
|the atmosphere over the arctic enough to affect||underline just started; why here?|
|the trajectories of ballistic missiles, induce||Ah, parallelism! It’s a list. To affect, (to) induce… so I’m looking for an “and” followed by a third infinitive verb.|
|electric currents that can cause blackouts in some areas and||Great, here’s the “and”|
|corrosion in north-south pipelines||Huh? Okay, corrosion clearly doesn’t fit with the first two – it’s not an infinitive verb…|
|Now I’m confused about the basic meaning. The northern lights affect missile trajectories, induce currents and cause corrosion? Is that what they’re trying to say but they didn’t put the verb in? Something seems weird. The first two are verbs – and we do need a verb to follow heat the atmosphere enough to do something else… but the third one is a noun and I can’t change it. It’s not underlined.|
Right about now, I noticed something that changed my entire view of the sentence. Initially, after seeing what looked like the first two items on a list, my instinct was to look for a list of three things: X, Y, and Z. By the end, though, I’m questioning my instinct because it seems like I have to have verb forms… and yet that third item, a noun, can’t be changed (since it isn’t underlined). What to do?
And this is when I notice a tiny little detail that opened up the problem for me. The structure for a list of three things is X, Y, and Z. We have to have a comma after the Y and before the and. But there’s no comma before the and in the original sentence! In other words that and corrosion piece is not intended to be the third item in a list of three things!
Wow. Okay, if that and is not meant to be the third item in the list, then what does it go with? The word and always indicates parallelism, so what is corrosion parallel to?
Oh, I think I get it now. Check this out:
“…electric currents that can cause blackouts in some areas and corrosion in north-south pipelines.”
The structure is actually “cause X and Y” where X = blackouts and Y = corrosion.
Where does that leave us with the earlier part of the sentence? I’m not sure. Maybe the first two things need an and in order to make them parallel (affect and induce). Or maybe the second item is supposed to be a modifier for the first, something like this: …affect the trajectories of missiles, inducing electric currents that can cause X and Y. I have no idea how they’ll try to fix this – time to check the other answers (and eliminate answer A)!
does it fix the problem?
|(B)||that (X) the trajectories…, (Y) induce…||No. Trajectories is a noun, but induce is a verb; they’re not parallel. Eliminate B.|
|(C)||that it (X) affects, (Y) induces… and?||No. Affects and induces are parallel but they have no and in between. We still can’t use and corrosion as the third item on the list, though, both because there’s no comma before the word and and because corrosion is a noun while affects and induces are verbs. Eliminate C.|
|(D)||that the trajectories (X) are affected and (Y) induces…||No. They inserted the word and between the two verbs, which is a good move, but are affected is plural (and matches the plural subject trajectories), while induces is singular (and so doesn’t match the plural subject). Eliminate D.|
|(E)||to (X) affect… and (Y) induce…||Bingo! They’ve inserted the word and between the two verbs and also made sure that the two are parallel (they’re both in the infinitive form).|
The correct answer is E.
The core structure of the correct sentence (E) is:
“Displays (of the aurora borealis) can heat the atmosphere enough to affect X and induce Y.”
That “Y” portion can be broken out in this way:
“electric currents that can cause M and N”
Note that the problem also tests a particular idiom: can heat X enough to Y. Answers B, C, and D all incorrectly replace the word to with the word that: can heat X enough that Y.
Be very careful when learning this idiom; in other circumstances, the word enough can be followed by the word that. This sentence is correct, for example: Studying is important enough that we should make it a priority. In our given sentence, though, we were trying to say that the atmosphere could be heated enough to cause another action to happen. The action should be in verb form: enough to <verb>.
Key Takeaways for Parallelism and Meaning
(1) Sometimes, the original sentence will seem to imply a certain structure or a certain way of reading it… only to fall apart as you continue to read the sentence. This might also mess up your understanding of the meaning of the sentence.
(2) If this happens, try to figure out why you had a certain expectation in the first place. I thought the sentence was going to have the X, Y, and Z structure, because the first two components (X, Y) were there. Once I looked for the third element (, and Z) and realized that the sentence couldn’t work that way, I was on my way to solving the problem.
(3) Little clues can sometimes make a big difference. In the real world, you will see lists written in two different ways: X, Y and Z or X, Y, and Z. What’s the difference? The presence or absence of the second comma. The GMAT always uses the second comma (the latter example), so if you think you might have a list of three things but something seems funny, look for that comma. If it’s not there, then that sentence or answer choice is not trying to give a list of three things.
* GMATPrep® questions courtesy of the Graduate Management Admissions Council. Usage of this question does not imply endorsement by GMAC.