The Chicago and Calumet Rivers originally flowed into the St. Lawrence

The Chicago and Calumet Rivers originally flowed into the St. Lawrence by way of Lake Michigan, but having been redirected by constructing canals so that the water now empties into the Mississippi by way of the Illinois River.

(A) Rivers originally flowed into the St. Lawrence by way of Lake Michigan, but having been redirected by constructing (-ing modifier is not correct)
(B) Rivers had originally flowed into the St. Lawrence by way of Lake Michigan, but they have been redirected by constructing (past perfect is unnecessary no other past even is mentioned for sequencing )
(C) Rivers, which originally flowed into the St. Lawrence by way of Lake Michigan but have been redirected by the construction of (fragmented sentence first part before but has no verb)
(D) Rivers, originally flowing into the St. Lawrence by way of Lake Michigan, but having been redirected by the construction of (same mistake as A, -ing modifer having bean is 99% wrong)
(E) Rivers, originally flowing into the St. Lawrence by way of Lake Michigan, have been redirected through the construction of (perfect S,ing,verb...)

only E follows correct grammatical construction.

There are very few "rules" to GMAT SC that apply to every single question without fail -- after all, it is mostly a reasoning test, not a do-you-know-the-rules test.

Generally speaking, the past perfect tense is used for an action or state of being that started in the past and then ended in the past. Some convenient indicators of the proper use of past perfect are past tense verbs. For example, "By the time my guests arrived, I had already eaten the whole cheese platter by myself."

But just because the past tense "marker" is nice to have, is that the only way in which the past perfect can be properly used?

In the first example you linked, let's ignore any other issues with the sentence and focus on the tense of the first action:

1) "Ozone, a special form of oxygen that screens out harmful ultraviolet rays, reaches high concentrations twelve miles above Earth, where it has long appeared that it was immune from human influence; we have now realized, though, that emissions of industrial chlorofluorocarbons deplete the ozone layer.

2) "Ozone, a special form of oxygen that screens out harmful ultraviolet rays, reaches high concentrations twelve miles above Earth, where it had long appeared immune to human influence; we have now realized, though, that emissions of industrial chlorofluorocarbons deplete the ozone layer."

In this sentence, it is clear that it no longer appears that the ozone is immune from human influence. In other words, the action of "appearing" started in the past and ended in the past. So, it is completely illogical to use the present perfect tense in this case. Instead, we have to rely on the past perfect, because that's the only other option in our answer choices.

Here, the GMAT forces our hand to accept the past perfect, even though there is no "marker" in the simple past tense. So, there is no ironclad rule that prevents this construction on every single question.

However, just because something is allowed on one question doesn't mean that it's the best option on an entirely different question. Here are (B) and (E) for the question on this thread:

In this version of the sentence, we have the past perfect "had originally flowed." Is it wrong to use this tense? Not necessarily. But is it the best option that we're given out of the available options? We'll have to see whether another answer choice conveys the timeline in a simple, clear manner.

More importantly, we have to investigate the -ing word "constructing." Who, exactly, is constructing canals in this answer choice? As written, this sentence implies that the Chicago and Calumet Rivers are constructing the canals. This is clearly illogical, and is a strong reason to eliminate (B).

Here's (E):

This neatly fixes both issues with (B). By changing "had originally flowed" to "originally flowing," the sentence gives us a clear and simple timeline of events. And by replacing "by constructing" with the more passive "through the construction of," (E) implies that someone else has constructed the canals, not the rivers themselves.

I hope that helps!

Let's start with a couple of examples:

• "The restaurant, simply called Tim's by the locals, has been the city's best kept secret for years." - Notice that "called" is just an -ed modifier, not a verb. It describes "the restaurant", and we know from the context that this modifier applies at present.
• "The restaurant, originally called Tim's Pizza, has been renamed Tim's Famous Original Pizza." - Again, "called" is an -ed modifier that describes "the restaurant". However, in this case, we know from the context that this modifier applies to the time BEFORE the restaurant was renamed.

Back to the official question: "flowing" is a modifier, NOT a verb. So "originally flowing" does NOT indicate the "present continuous" or any other tense. It's simply a modifier, and the word "originally" tells us that this modifier applies to the time BEFORE the rivers were redirected.

In short, "-ed" and "-ing" modifiers do not have an inherent tense. Instead, the time frame of such modifiers is indicated by the context.

I hope that helps!

IMO E. Clearly the choices are between B and E. A, C, D are improper sentence constructions in itself.

Now for B, past perfect (had) is not appropriate. There is no past tense later in the sentence to which this event can relate to; instead present perfect (have) is used. So this is incorrect. Possibly "had" could have been correct in case it was written "rivers had originally flowed into ..., but then their course was diverted by..."

That leaves E - which seems to have no flaws.

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Please give Kudos if you like my explanation. Thank you

the only error in B is "constructing canal"

by doing is used to show the method the subject uses to do the action in main clause. there is no reason grammatically to use constructing here.

second, b seem that they have been redirected by canal, not by construction of cannal, which is the intended meaning.

I marked B because redirected through looks wrong idiom. Can someone confirm this?

E is my Take as well.

B Rivers had originally flowed into the St. Lawrence by way of Lake Michigan, but they have been redirected by constructing . Had and constructing are spoiling the show

https://www.youtube.com/watch?v=R9rcv8rN0EM&list=PL0LGZMx8pP21Jx8Er_L5V-HfbPn9DKY7M&index=12

26:16

Can some expert please help me with a simple and understandable explanation of this sentence and the eliminated choices.?

Apart from the tenses issue in B, to say rivers have been redirected BY constructing the canals seems improper. It's better to say they have been redirected through the construction canals (E)
Just listen to the meaning of both.

@experts pls correct me if this wrong.

Dear GMATNinja,

According to RonPurewal (https://www.manhattanprep.com/gmat/foru ... 36-75.html):


Ron implies that Verb-ing modifiers usually happen at the same time as the main verb. In other words, Verb-ing modifiers take the tense of the main verb.

What do you think of the above opinion?

Dear GMATNinja,

More detail of RonPurewal 's stance on this (https://www.manhattanprep.com/gmat/foru ... ml#p108571):

Dear GMATNinja,

I agree with your 2 examples.

Here is OA : Fossils of the arm of a sloth, found in Puerto Rico in 1991, have been dated at 34 million years old, making the sloth the earliest known mammal on the Greater Antilles Islands.


FOUND happened in 1991. HAVE BEEN denotes the present.

However, I think the question at hand is about VERB-ING, not VERB-ED, modifier.

Dear GMATNinja,


I think GMATGuruNY also agrees with RonPurewal. Both advocate that COMMA + VERBing needs to happen AT THE SAME TIME AS the main verb.

I'm confused here

Again, the word originally makes all the difference in the world here. Without it, we would certainly have an issue -- it would seem as though the rivers are simultaneously flowing into the St. Lawrence and the Illinois/Mississippi Rivers, and that wouldn't make any sense (that's why "earning" was an issue in the other example you referenced).

The word "originally" allows us to understand the timing of the events with no ambiguity... the context is vital, and that's why this question and the other one that you referenced must be analyzed differently.

As I always say, there are very few concrete, black and white "rules" that you can blindly apply to GMAT SC. Just because something works in one correct answer choice doesn't mean that it's always correct. And just because something doesn't work in an incorrect answer choice doesn't mean that it's always wrong. That's why comparing sentences from different problems is rarely productive.

Also, there's a convenient "edit" button on the GMAT Club forum, so if you have more things to say about the same question, you can add to your original post -- so there's no need to write five separate posts on the same thing.

I hope that helps!

if we read only original choice and we understand the intended meaning, everybody pass gmat and get into havard

when reading choice a, we need first to try to find the intended meaning. normally you do not understand anything. but if we focus on verb/action we can know how many action/verb in the choice a and you can guess the correct meaning relation between/among verb/ideas. correct meaning relation is the intended meaning. dont worry, if you guess wrongly because there is another choice , the official answer there, which offer correct meaning relation.

the critical step is to guess/find out the correct meaning relation between/among ideas/verbs and we perform this step immediately when we read choice a. normally, you can eliminate 2 or 3 choices easily because of structure problems. when faced with 2 remaining choice, we have to solve meaning problem. solving meaning problem means we have to find out the correct meaning relation between/among ideas /verbs. it is better to try to find the correct meaning relation. dont worry if you can not find the correct relation when you read choice A. the key skill is when you read choice a and other choice, focus on finding out the correct meaning relation.

be prepared to find out the correct meaning relation when you begin to solve sc.

1. read choice a, read by chunks, realize grammatical roles of each chunk when you read . realize 2 or 3 ideas/verb. guess the correct meaning relation between/among these verbs/ideas immediately . if you can not find the correct meaning relation, read other choice
2. read other choices, realize the correct meaning relation.

applying those skills.
read choice a, realizing two ideas/verbs, which are "flow" and "redirect" . guess the correct relation. the correct relation could be the river at first flow then is directed.
with this guess in mind, read other choices and confirm our guess.

Nice question.

The Chicago and Calumet Rivers, [strike]originally flowing into the St. Lawrence by way of Lake Michigan[/strike], have been redirected through the construction of canals so that the water now empties into the Mississippi by way of the Illinois River.

E it is.