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


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.

Option A
Option B
Option C
Option D
Option E

(This question is from Official Guide. Therefore, because of copyrights, the complete question cannot be copied here. The question can be accessed at GMAT Club)


Sentence Analysis

To map Earth’s interior, geologists use a network of instruments to chart some waves. (Please note that I’m deliberately substituting technical words with generic versions since this is how I read, and I have seen that it works)

The sentence then presents additional information about waves. These waves originate in the earth’s crust and ricochet around its interior. The sentence then provides additional information about how the waves ricochet: They travel

  1. most rapidly through cold, dense regions
  2. slowly through hotter rocks

The main problem with the sentence is the placement of “most rapidly”. It should appear after “traveling” since “traveling” is common to both “most rapidly” and “slower”.

Also, since we are using an adverb form with ‘ly’ (rapidly) in the first element of the list, it is preferable to use a similar version (slowly) in the second element too. Or we could have used faster and slower in the two elements. Please note that this is just a stylistic preference, not a deterministic error for which alone you can reject an option statement.

Option Analysis

(A) Incorrect. For the problems listed above.

(B) Incorrect. ‘which’ at the end of a clause cannot refer to the subject of the clause; it refers to the closest noun, which in this case is ‘interior’. However, such a reference will produce illogical meaning.

(C) Correct. Both the problems of the original sentence have been corrected in this option.

(D) Incorrect. For the following reasons:

  1. By presenting “travel” on the same level as “originate” and “ricochet”, this option introduces grammatical and logical errors. Grammatically, we have two “and” in the same list. Logically, “traveling” is not parallel to “ricochet”; “travelling” and the corresponding information provides additional information about how the waves ricochet around the earth’s interior.
  2. The way “cold, dense regions, and slower” is written (first two elements separated by a comma and then comma+and before slower), it seems these three elements are in a list. Clearly, illogical. Also, even if we want to take “slower” parallel to “most rapidly”, we cannot since “travel”, which must be common to both ‘slower’ and ‘most rapidly’, appears after “most rapidly”.

(E) Incorrect. By presenting the information “that travel most rapidly…” as a separate dependent clause, the sentence distorts the original meaning in which this information, quite logically, provides additional information about how the waves ricochet around the earth’s interior. Also, as explained in the sentence analysis, ‘slowly’ is preferred over ‘slower’ in this context.

About Chiranjeev Singh

An Alumnus of IIM Ahmedabad and with scores of 780 (2017) and 770 (2013) on GMAT and 99.98%ile on CAT, Chiranjeev is one of the most qualified GMAT tutors in India. Chiranjeev has earlier served as Director of Curriculum at e-GMAT. Chiranjeev has been helping students ace GMAT since 2012. He follows a concept-based methodology to teaching GMAT and is very committed to student success. You may contact him for any private GMAT tutoring needs at He conducts online sessions for students across the world.

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