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

Hello GMATNinja,

I eliminated option C because the "more slowly" did not make sense to me.
The waves move most rapidly through A and "more slowly" through B.
I figured we'd need something to compare to here, something that was moving "less slowly", in order to warrant the use of "more slowly".
For example, "The snail moved more slowly than the sloth".
I understand that the "most rapidly" does refer to less slow movement, but this effectively means the waves moved more slowly through B than as quickly as they did through A.
Not sure how well I'm explaining this, but could you please help clear the air and explain why this usage is okay? Option C just seemed off to me because of this.

I still do not understand this. Isn’t that modifying seismic waves, thus there is no clause as per what the experts are saying?? Pls help!’

I still do not understand this. Isn’t that modifying seismic waves, thus there is no clause as per what the experts are saying?? Pls help!’

Posted from my mobile device

I still do not understand this. Isn’t that modifying seismic waves, thus there is no clause as per what the experts are saying?? Pls help!’

Posted from my mobile device

Normally we wouldn't stack two noun modifiers together like that, regardless of whether "which" or "that" is used. In this case, since the first modifier ends in a noun ("interior"), the second modifier ends up incorrectly modifying that noun. That's one of the main reasons we're not going to do well stacking modifiers.

In theory, you could have two modifiers, one essential (using that) and the other non-essential (using which):

The book that I wrote, which comes out this summer, describes my experiences in Iran.

Note that in this case, the first modifier ends in a verb, not a noun, so there's no confusion about what the second noun is modifying.

While the explanation in this post by DmitryFarber is really great as it explains the underlying concept, I am a bit confused. I would highly appreciate if any expert can please spare a couple of minutes to shed some light on this and validate my understanding.

The second modifier "which travel most..." doesn't have a plural verb "travels" after "which", so I think "interior" cannot be a referent for "which" in the first place. Now, as it reduces the ambiguity, I further rolled back my eyes to find the next preceding noun "seismic waves" that can be a suitable referent for "which". Now, if I check the meaning as well, it makes perfect sense to have "seismic waves" as the referent for "which".

This implies, "which" is UNAMBIGUOUSLY referring to "seismic waves" in option B. Why I am saying it is unambiguous? I am referring to the example that shows that "which" is UNAMBIGUOUSLY referring to "book", even though there is a "that" CLAUSE interfering in between the noun and the "which".

What confuses me more when I read explanations in this thread is that they mention that option B is straightaway wrong as "interior" cannot be the referent of "which" as it is non-sensical, and it is "sesimic waves" that "travel". Hence, Option B is eliminated. While there are so many official examples in which "which" is not IMMEDIATELY followed by its logical "noun" referent, I think it makes sense to PAUSE and understand the underlying concept once if possible.

The possible explanation for rejecting option B could be that it is not as elegant, though grammatically not incorrect, as option C. I also came across the GMATNinja's post that I believe is trying to convey the same thing. But I am not so sure. Hence, some clarity would surely help me.

Hello Pankaj0901,

We hope this finds you well.

To answer your query, the subject-verb disagreement you have referenced with regards to Option B does not indicate that "which" refers to "waves"; "which" refers to "interior" in Option B, and this disagreement is just another error in this modification.

We hope this helps.
All the best!
Experts' Global Team

I don't think (E) necessarily implies that there are two kinds of seismic waves. To illustrate why, consider this example:

"Tim enjoys music (1) that is very loud and (2) that lacks originality."

Both (1) and (2) describe the kind of music that Tim enjoys. If we instead wanted to emphasize that Tim likes two DISTINCT buckets of music, it would be better to write, "Tim enjoys (1) music that is very loud and (2) music that lacks originality."

Similarly, if we wanted to suggest that there are two types of seismic waves, that meaning would be more clearly expressed with, "... geologists use a network of seismometers to chart (1) seismic waves that originate in the earth's crust and ricochet around its interior and (2) seismic waves that travel most rapidly through..."

As for the parallelism...

• Yes, (C) certainly looks/feels/sounds much better with the symmetrical set of adverbs ("most rapidly" and "more slowly").
• But instead of analyzing the parallelism in (E) from a purely technical standpoint, try thinking about what the word "slower" suggests from a meaning perspective: it implies that there is some sort of comparison, and leaves the reader wondering, "Slower than what or when?"
• For example, it would be okay to write something like, "... waves that travel slower through hotter rocks THAN through cold, dense regions," but that's not what we have in (E).
• So yes, "more slowly" certainly looks/feels/sounds more parallel in this case. But more importantly, it's more appropriate from a meaning perspective.

(C) is definitely the better choice. (For another vote against (E), notice that "and" shows up THREE times -- this isn't necessarily WRONG, exactly, but makes it awfully hard for the reader to keep all of the parallelism straight.)

I hope that helps!

Hi Pankaj0901!

You're right that we definitely have some leeway in applying noun modifiers. The "touch rule" is not an absolute rule. Sometimes we can skip over some modifiers to refer back to an earlier noun, as long as the intent is clear. Here's a case with a few modifiers to skip over (but be aware of the spoiler--it's from an official mba.com exam): https://gmatclub.com/forum/although-she ... ml#p357857

There's even an OG question on which we have to skip over a verb to apply a "which" to the previous noun:
https://gmatclub.com/forum/gusty-wester ... l#p1869067

Now, having said all that, this in no way means that we can just go back and apply the modifier to whatever noun we want. Even this simple sentence is wrong:
My dad wrote a book, who is an expert on poetry.

What's wrong with this? Clearly, the only reasonable referent for WHO is "dad," right? And we're just skipping over the verb and object to refer to the main subject, right? No big deal? Nope. Can't do it. This is really saying that my book is an expert on poetry (and a "who," apparently). Why? First, if we wanted to modify "dad," there's an easy way to do it--just put the modifier next to "dad." Second, even though the clause is short and simple, we have still jumped to the end. This muddies our intent and makes the reader work too hard. We should only attach a modifier as far away from a noun as we NEED to in order to get a clear and logical sentence. In this case, using an adverbial modifier is a clean, easy way out of this problem, and that's a solution we see all the time in SC.

In my earlier comments, I also mentioned that we don't normally stack two noun modifiers on top of each other, and that's another reason to avoid the construction in C. If we wanted to keep modifying waves, then there would be an easy way to do that, too. We could say "waves THAT originate . . . AND THAT travel." Since the author doesn't do that, they are clearly signaling that they are doing modifying waves. That may be one reason that so many of us state so plainly that "which" must apply (incorrectly) to "interior."

Now, there's also a more important reason to go with C. The adverbial modifier is really what we wanted all along! The point of the sentence is not just "Hey, scientists used these waves, and by the way, the waves have variable speed." The point is that we can use the waves to make seismic maps BECAUSE the waves travel at different speeds through different materials. The adverbial -ing modifier conveys this meaning, and the noun modifier does not.

Finally, be careful not to let the letter S fool you on singular vs. plural. Remember that an S at the end of a verb usually makes it singular, not plural! I know you were using the right grammar in matching "travel" with a plural noun, but watch that you don't get mixed up in the terminology. When we say "waves travel," both the noun and the verb are plural, and when we say "the wave travels," both the noun and the verb are singular. Keeping this distinction will help make all our explanations clearer.

A present participle modifier refers to the subject of the preceding clause or the entire preceding clause.

In this case the clause " that originate in the earth's crust and ricochet around its interior" is the preceding clause, and hence the present participle modifier "traveling most rapidly...." refers to the subject "that" of the previous clause; the pronoun "that" here is used to replace "waves".

Consider that the entire present participle modifier "traveling most rapidly...." is nested within the relative clause "that originate in the earth's crust and ricochet around its.......through hotter rocks."

You are right in thinking that there could be a bit of ambiguity since the present participle clause "travelling...." could refer to "Geologists", if one considers that it is outside the relative clause "that originate in the earth's crust and ricochet around its.......through hotter rocks." However in that case we would have to choose E as the correct answer, which has a more severe problem as follows:

E is wrong because two clauses are joined just with "and" not with comma + "and".

Wrong: I play and I sing
Right: I play, and I sing
Right: I play and sing.

Similarly,
Wrong: that originate and ricochet and that travel
Right: that originate and ricochet, and that travel
Right: that originate, ricochet and travel

Moreover option E does not depict the bearing between ricocheting and travelling and considers them as two different activities.

GMATNinja

In choice (E), there is no need to add comma before and that, right? >>> interior "," and that travel most rapidly through cold, dense regions and slower
I mean that comma is not another key to eliminate this choice

GMATNinja
I am not exactly sure where to find this comma ING usage in more detail.
Could you help point out the difference between a compound clause that come before the comma ING. In some cases, I am not sure if the comma ING will actually modify more than just the immediate preceding as shown in this SC. Please see AC-C in this example? The use of AND seems to allow the "the politician decided to postpone" to be modified by the comma ING. However, most responses in that thread showed that distorted the meaning. My question is if you could confirm the compounded structure in this example is what allowed both clauses to be modified, in comparison to the OP of this Sc, in which, only the object of the main clause (seismic waves) is modified.
For instance, https://gmatclub.com/forum/finally-reac ... 60019.html

A present participle modifier refers to the subject of the preceding clause or the entire preceding clause.

In this case the clause " that originate in the earth's crust and ricochet around its interior" is the preceding clause, and hence the present participle modifier "traveling most rapidly...." refers to the subject "that" of the previous clause; the pronoun "that" here is used to replace "waves".

Consider that the entire present participle modifier "traveling most rapidly...." is nested within the relative clause "that originate in the earth's crust and ricochet around its.......through hotter rocks."

You are right in thinking that there could be a bit of ambiguity since the present participle clause "travelling...." could refer to "Geologists", if one considers that it is outside the relative clause "that originate in the earth's crust and ricochet around its.......through hotter rocks." However in that case we would have to choose E as the correct answer, which has a more severe problem as follows:

E is wrong because two clauses are joined just with "and" not with comma + "and".

Wrong: I play and I sing
Right: I play, and I sing
Right: I play and sing.

Similarly,
Wrong: that originate and ricochet and that travel
Right: that originate and ricochet, and that travel
Right: that originate, ricochet and travel

Moreover option E does not depict the bearing between ricocheting and travelling and considers them as two different activities.

Right: that originate, ricochet and travel

Right: that originate and ricochet, and that travel

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