To map Earth's interior, geologists use a network of seismometers to

https://discovermagazine.com/1996/nov/hotandcoldspotsb935

To map Earth’s interior, geologists use a worldwide network of seismometers that chart the movement of seismic waves generated by earthquakes. These waves, originating in Earth’s crust or upper mantle, ricochet around the interior, traveling most rapidly through cold, dense regions, and more slowly through hotter rocks.

First glance

The first three choices place a comma after the word interior. The final two choices remove the comma and use the word and. These two clues together signal possible Modifier, Sentence Structure, or Parallelism issues.

Issues

(1) Parallelism: X and Y

Find the X and Y portions that map to the parallelism marker.

(A) most rapidly traveling … and slower

(B) travel most rapidly … and more slowly

(C) traveling most rapidly … and more slowly

(D) most rapidly travel … and slower

(E) travel most rapidly … and slower

Answers (B) and (C) are definitely parallel because both use the –ly versions of the relevant words (rapidly, slowly). Answers (A), (D), and (E) use slower instead of more slowly. Slower is primarily an adjective, not an adverb. While it can be possible in certain circumstances to use slower as an adverb, when parallelism is in play, it’s better to use the strict adverb form to signal clear parallelism: rapidly and slowly. Eliminate choices (A), (D), and (E).

(2) Modifier: which

Answer (B) employs a comma-which modifier. Check to make sure it’s used correctly.

A comma-which modifier should refer back to the closest main noun before the comma. In this case, that noun is interior, which does not make sense. The prior noun is earth’s crust, but this also does not make sense. The logical word is seismic waves but this is too far back to go with the comma-which modifier. Eliminate choice (B).

The Correct Answer

Correct answer (C) uses the parallel construction most rapidly and more slowly. It also properly uses a comma –ing modifier, which refers back to the prior action, not just the prior main noun. The prior action is the seismic waves ricochet(ing) around the interior of the earth’s crust.