Hello Everyone!
Let's take a closer look at this question, one issue at a time, and narrow it down to the correct answer! First, here is the original question with any major differences between the options highlighted in orange:
To map Earth's interior, geologists use a network of seismometers to chart seismic waves that originate in the earth's crust and ricochet around its interior, most rapidly traveling through cold, dense regions and slower through hotter rocks.
(A) interior, most rapidly traveling through cold, dense regions and slower
(B) interior, which travel most rapidly through cold, dense regions, and more slowly
(C) interior, traveling most rapidly through cold, dense regions and more slowly
(D) interior and most rapidly travel through cold, dense regions, and slower
(E) interior and that travel most rapidly through cold, dense regions and slower
After a quick glance over the options, there are a few places we can focus on:
1. most rapidly traveling / travel most rapidly / traveling most rapidly / most rapidly travel
2. slower / more slowly
3. comma vs. no comma before "and"
Since #1 on our list seems a bit complicated, let's start with #2 on our list because it should eliminate 2-3 options quickly. This is an issue of parallelism, so let's make sure the two adverbs are written using the same structure:
(A) interior, most rapidly traveling through cold, dense regions and slower
(B) interior, which travel most rapidly through cold, dense regions, and more slowly
(C) interior, traveling most rapidly through cold, dense regions and more slowly
(D) interior and most rapidly travel through cold, dense regions, and slower
(E) interior and that travel most rapidly through cold, dense regions and slower
We can eliminate options A, D, & E because they don't use parallel structure when describing the two speeds a seismometer travels in different conditions.
Now that we have it narrowed down to only 2 options, let's see if we can find any other problems to eliminate one of these options, leaving us with the correct choice:
(B) interior, which travel most rapidly through cold, dense regions, and more slowly
This is INCORRECT for two reasons. First, using the comma + which creates a modifier that must refer back to the nearest preceding noun. In this case, that noun is "interior," which isn't what's traveling - the seismometer is! Second, there shouldn't be a comma before the "and" here. It's connecting two dependent clauses together, so it doesn't need a comma. We only use comma + and when connecting two independent clauses or when introducing the last item in a list. Since this is neither of those instances, it's not necessary.
(C) interior, traveling most rapidly through cold, dense regions and more slowly
This is CORRECT! It's clear that the phrase that begins with "traveling" is referring back to the seismometer, it uses parallel structure (rapidly/slowly), and it doesn't have that extra comma before "and."
There you have it - option C is the correct choice!
Don't study for the GMAT. Train for it.
