Hello Everyone!
This is a great example of a GMAT question that focuses on parallelism and subject-verb agreement! Let's start by taking a quick look at the original question, and highlighting any major differences between the options in orange:
Scientists have recently found evidence that black holes-regions of space in which matter is so concentrated and the pull of gravity so powerful that nothing, not even light, can emerge from them-probably exist at the core of nearly all galaxies and the mass of each black hole is proportional to its host galaxy.
(A) exist at the core of nearly all galaxies and the mass of each black hole is proportional to
(B) exist at the core of nearly all galaxies and that the mass of each black hole is proportional to that of
(C) exist at the core of nearly all galaxies, and that the mass of each black hole is proportional to
(D) exists at the core of nearly all galaxies, and that the mass of each black hole is proportional to that of
(E) exists at the core of nearly all galaxies and the mass of each black hole is proportional to that of
After taking a quick glance over the options, 2 major differences jumped out:
1. exist vs. exists (subject-verb agreement)
2. to vs. to that of (parallelism)
Let's start with #1 on our list: subject-verb agreement. If we look at the entire sentence, we should be able to identify the subject that matches up with the verb "exist/exists" pretty quickly:
Scientists have recently found evidence that black holes-regions of space in which matter is so concentrated and the pull of gravity so powerful that nothing, not even light, can emerge from them-probably exist at the core of nearly all galaxies and the mass of each black hole is proportional to its host galaxy.
Since the subject "black holes" is plural, we need to make sure the verb we use is also plural:
(A) exist at the core of nearly all galaxies and the mass of each black hole is proportional to
(B) exist at the core of nearly all galaxies and that the mass of each black hole is proportional to that of
(C) exist at the core of nearly all galaxies, and that the mass of each black hole is proportional to
(D) exists at the core of nearly all galaxies, and that the mass of each black hole is proportional to that of
(E) exists at the core of nearly all galaxies and the mass of each black hole is proportional to that of
We can eliminate options D & E because the singular verb "exists" doesn't match the plural subject "black holes." Pretty easy, right?
Now let's tackle #2 on our list: parallelism. This sentence is comparing two items, and we need to make sure that they are parallel in type. To make this easier to spot, we'll add the rest of the comparison to each option for you. Here's how they break down:
(A) exist at the core of nearly all galaxies and the mass of each black hole is proportional to its host galaxy.
This is INCORRECT because it's comparing mass to galaxy, which aren't the same thing at all!
(B) exist at the core of nearly all galaxies and that the mass of each black hole is proportional to that of its host galaxy.
This is CORRECT! It's clear that by adding in the phrase "that of," we're now comparing the mass of each black hole to the mass of its host galaxy, which is parallel.
(C) exist at the core of nearly all galaxies, and that the mass of each black hole is proportional to its host galaxy.
This is INCORRECT because it's trying to compare mass to galaxies, which aren't parallel.
There you have it - option B is the correct choice! It uses proper parallel structure to compare two items, and it doesn't have any subject-verb agreement issues!
Don't study for the GMAT. Train for it.
