You're right to be confused, Mukherjee.

This seems to be another of those "pick the least bad among the answers." Interestingly enough, if you wrote any of these 5 choices in a composition in college, I suspect your English professor would be more likely to ding you for an ambiguous pronoun reference than for some of the other mistakes. Just goes to show that the mistakes the GMAT considers the most serious aren't necessarily the ones that would be considered the worst in real life. I guess it's no wonder so many people think standardized tests are kind of worthless.

Anyway, B) is the easiest to eliminate, since the second clause is just a fragment. Also, in E), "much" can't modify "variables." For the remaining choices (which are very tricky), look for issues of parallelism to eliminate two of them.

Hope this helps.

OA: Many of today's mathematicians use computers to test CASES that would either take too much time or involve too many variables to test manually, ALLOWING them to explore the theoretical issues that were impossible to test a generation ago.

Here, allowing serves as a COMMA + VERBing modifier.
On the GMAT, the agent of a COMMA + VERBing modifier must be the NEAREST preceding subject.
Here, the subject that most closely precedes allowing is cases:
CASES that would either take too much time or involve too many variables to test manually, ALLOWING them to explore theoretical issues.
The implication is that CASES that would take too much time would be responsible for ALLOWING them to explore theoretical issues -- not the intended meaning.
For this reason, be skeptical of this OA.

An analogous SC in the OG12:
The Army Corps of Engineers proposed building parallel to shore A BREAKWATER OF ROCKS that would rise six feet above the waterline and act as a buffer, ABSORBING the energy of crashing waves and protecting the beaches.

Here, absorbing serves as a COMMA + VERBIng modifier.
The nearest preceding subject is a breakwater of rocks.
This construction implies -- correctly -- that a breakwater of rocks would be responsible for absorbing the energy of crashing waves:
A BREAKWATER OF ROCKS that would rise six feet above the waterline and act as a buffer, ABSORBING the energy of crashing waves.
Conveyed meaning:
When a breakwater of rocks would rise and act as a buffer, it would at the same time be ABSORBING the energy of crashing waves.

GMATGuruNY wrote: For this reason, be skeptical of this OA.

Yes, the whole question kind of stinks.

GMATGuruNY wrote: On the GMAT, the agent of a COMMA + VERBing modifier must be the NEAREST preceding subject.

I was reading through your explanation a little more, GMATGuruNY, and I have a couple of questions for you:

1) Do the writers of the GMAT explicitly state that the "nearest preceding subject" must be the agent of such a participle?

2) What exactly do you mean by "nearest preceding subject"? In the first sentence, "cases" is the direct object of an infinitive; and in the second, "breakwater" is the direct object of a gerund. Do you mean "nearest preceding noun that would make sense as a subject"?

Even I have the same doubts!

neptune28 wrote:

GMATGuruNY wrote: On the GMAT, the agent of a COMMA + VERBing modifier must be the NEAREST preceding subject.

I was reading through your explanation a little more, GMATGuruNY, and I have a couple of questions for you:

2) What exactly do you mean by "nearest preceding subject"? In the first sentence, "cases" is the direct object of an infinitive; and in the second, "breakwater" is the direct object of a gerund. Do you mean "nearest preceding noun that would make sense as a subject"?

A word can serve more than one purpose.

The Army Corps of Engineers proposed building parallel to shore A BREAKWATER OF ROCKS that would rise six feet above the waterline and act as a buffer, ABSORBING the energy of crashing waves and protecting the beaches.
Here, a breakwater of rocks serves not only as the direct object of building but also as the SUBJECT of the verbs would rise and would act.
What WOULD RISE six feet above the waterline and ACT as a buffer?
A BREAKWATER OF ROCKS.

Many of today's mathematicians use computers to test CASES that would either take too much time or involve too many variables to test manually, ALLOWING them to explore the theoretical issues that were impossible to test a generation ago.
Here, cases serves not only as the direct object of to test but also as the subject of the verbs would take and would involve.
What WOULD TAKE too much time and INVOLVE too many variables?
CASES.

Note the following:
Every that-clause has a SUBJECT.

a breakwater of rocks that would rise six feet above the waterline and act as a buffer
Here, that refers to a breakwater of rocks.
Implication:
A BREAKWATER OF ROCKS serves as the SUBJECT of the verbs in the that-clause (would rise and would act).

cases that would either take too much time or involve too many variables to test manually
Here, that refers to cases.
Implication:
CASES serves as the SUBJECT of the verbs in the that-clause (would take and would involve).

1) Do the writers of the GMAT explicitly state that the "nearest preceding subject" must be the agent of such a participle?

We have to rely on precedent.

From the OG12:

SC30:
ANIMAL-HIDE SHIELDS with wooden frames were essential items of military equipment, PROTECTING warriors against enemy arrows and spears.
Here, the ANIMAL-HIDE SHIELDS are PROTECTING warriors.

SC47:
FIVE FLEDGLING SEA EAGLES left their nests in western Scotland this summer, BRINGING to 34 the number of wild birds successfully raised since transplants from Norway began in 1975.
Here, the FIVE FLEDGLING SEA EAGLES are BRINGING to 34 the number of wild birds successfully raised.

SC65:
A BREAKWATER OF ROCKS that would rise six feet above the waterline and act as a buffer, ABSORBING the energy of crashing waves and PROTECTING the beaches.
Here, a BREAKWATER OF ROCKS is ABSORBING and PROTECTING.

SC94:
The recent surge in the number of airplane flights has clogged the nation's air-traffic control system, LEADING to a 55-percent increase in delays at airports and PROMPTING fears among some officials that safety is being compromised.
Here, the RECENT SURGE is LEADING to an increase in delays and PROMPTING fears.

From GMAT Prep:
Among lower paid workers union members are less likely than non-union members to be enrolled in lower end insurance plans that impose stricter limits on medical services and require DOCTORS to see more patients, SPENDING less time with each.
Here, spending refers to doctors, which serves not only as the direct object of requiring but also as the SUBJECT of to see.
Conveyed meaning: the DOCTORS are SPENDING less time with each patient.

In every case, the agent of the COMMA + VERBing action is the nearest preceding subject.
The consistency of these SCs suggests a rule: on the GMAT, the agent of a COMMA + VERBing modifier should be the nearest preceding subject.

A word can serve more than one purpose.
Here, a breakwater of rocks serves not only as the direct object of proposed but also as the SUBJECT of the verbs would rise and act

Thanks for the explanation. Actually, "breakwater of rocks" is the direct object of the gerund "building." And technically speaking, the subject of the relative clause there is not "breakwater of rocks," but rather the relative pronoun "that." "Breakwater of rocks" is the antecedent. But anyway, I think I get what you're saying--that the agent is either the nearest preceding subject or the nearest antecedent of a relative clause for which the relative pronoun is the subject.

Note the following:
Every that-clause has a SUBJECT.

Yes, but what you're referring to is actually termed the "antecedent." For instance, in the following sentence:

The cats that I liked were obnoxious.

the subject of the relative clause is I.

Switching it around a bit:

The cats that scratched me were obnoxious.

the subject of the relative clause now becomes that. In both cases, cats is the antecedent.

The consistency of these SCs suggests a rule: on the GMAT, the agent of a COMMA + VERBing modifier should be the nearest preceding subject.

Great--thanks for the examples. And as long as no GMAT life forms have ever been discovered that violate that rule, then it sounds like a safe one.

Anyway, it seems pretty obvious that the original question posted here could use some serious professional help.

neptune28 wrote: And technically speaking, the subject of the relative clause there is not "breakwater of rocks," but rather the relative pronoun "that." "Breakwater of rocks" is the antecedent. But anyway, I think I get what you're saying--that the agent is either the nearest preceding subject or the nearest antecedent of a relative clause for which the relative pronoun is the subject.

a BREAKWATER OF ROCKS that would rise six feet above the waterline and act as a buffer.
Here, that does indeed serve as the subject of the relative clause.
The referent for that is a breakwater of rocks.
Since that refers to a breakwater of rocks, the conveyed meaning is that a BREAKWATER OF ROCKS would rise and would act.

Note the following:
Every that-clause has a SUBJECT.

Yes, but what you're referring to is actually termed the "antecedent." For instance, in the following sentence:

The cats that I liked were obnoxious.
the subject of the relative clause is I.

This structure differs from the one above.
Here, that serves not as the subject but as the DIRECT OBJECT.
Since the referent for that is the cats, the conveyed meaning is as follows:
I liked THE CATS.
While this that-clause still has a subject -- I -- the purpose of that is to stand in not for the subject but for the direct object.

GMATGuruNY wrote: a BREAKWATER OF ROCKS that would rise six feet above the waterline and act as a buffer.
Here, that does indeed serve as the subject of the relative clause.
The referent for that is a breakwater of rocks.
Since that refers to a breakwater of rocks, the conveyed meaning is that a BREAKWATER OF ROCKS would rise and would act.

Yes, that's definitely the conveyed meaning. However, I just wanted to point out that it's important to use the standard grammatical terms here in order to avoid confusion, because "the nearest preceding subject" is just too vague/imprecise IMO for most people to understand.

This structure differs from the one above.
Here, that serves not as the subject but as the DIRECT OBJECT.
Since the referent for that is the cats, the conveyed meaning is as follows:
I liked THE CATS.
While this that-clause still has a subject -- I -- the purpose of that is to stand in not for the subject but for the direct object.

That's essentially what I was saying, even though I didn't specifically mention that "that" is the direct object there because it seemed pretty straightforward. My point was also that in both cases, cats is the antecedent (of the relative clauses), never the subject or direct object per se, even though it basically "picks up" those meanings.

Anyway, I think we're pretty much on the same wavelength, and I suspect it's somewhere on the visible spectrum.

gmat guru said:On the GMAT, the agent of a COMMA + VERBing modifier must be the NEAREST preceding subject.
Here, the subject that most closely precedes allowing is cases:
CASES that would either take too much time or involve too many variables to test manually, ALLOWING them to explore theoretical issues.
The implication is that CASES that would take too much time would be responsible for ALLOWING them to explore theoretical issues -- not the intended meaning.
For this reason, be skeptical of this OA.

thanks guru for all the detailed explanation .kindly tell me if i can interpret this rule as follows : i believe that comma + verbing modifier must modify the previous clause and also should make sense with the subject of the previous clause .now if we look into the following sentence :The Army Corps of Engineers proposed building parallel to shore A BREAKWATER OF ROCKS that would rise six feet above the waterline and act as a buffer, ABSORBING the energy of crashing waves and protecting the beaches.

in this sentence the clause before comma is the "that" clause and hence the verb ing modifier must make sense with the subject of "that" clause and also should modify "that" clause only .
the important point to note is that in the above sentence the subject "The Army Corps of Engineers" is not the one getting modified by comma + verb ing modifier .
kindly tell me if my line of interpretation is correct

aditya8062 wrote:
thanks guru for all the detailed explanation .kindly tell me if i can interpret this rule as follows : i believe that comma + verbing modifier must modify the previous clause and also should make sense with the subject of the previous clause .now if we look into the following sentence :The Army Corps of Engineers proposed building parallel to shore A BREAKWATER OF ROCKS that would rise six feet above the waterline and act as a buffer, ABSORBING the energy of crashing waves and protecting the beaches.

in this sentence the clause before comma is the "that" clause and hence the verb ing modifier must make sense with the subject of "that" clause and also should modify "that" clause only .
the important point to note is that in the above sentence the subject "The Army Corps of Engineers" is not the one getting modified by comma + verb ing modifier .
kindly tell me if my line of interpretation is correct

Correct!

thanks a lot GMAT GURU .kindly help me to understand one more thing regarding comma + verbing modifier
if i summarize my previous points then i can say that for comma +verb ing modifier to work following 2 conditions are to be met :
1) the comma + verb ing modifier must modify the previous clause
2) the comma +verb ing modifier must make sense with the subject of the previous clause

now my question is is it always possible to satisfy condition no #2
i am asking this because at times i have found that condition no #2 does get violated
for instance consider the following sentence : FIVE FLEDGLING SEA EAGLES left their nests in western Scotland this summer, BRINGING to 34 the number of wild birds successfully raised since transplants from Norway began in 1975

in this sentence the subject "FIVE FLEDGLING SEA EAGLES" does not make sense with the verb ING modifier .i do agree that the comma + verb ing modifier does modify the previous clause though .

so can i assume that at times i might find sentences in which condition #2 might get violated?

also plz tell me that if my above analysis is correct then is this sentence correct :Crime has recently decreased in our neighborhood, leading to a rise in property values

aditya8062 wrote:thanks a lot GMAT GURU .kindly help me to understand one more thing regarding comma + verbing modifier
if i summarize my previous points then i can say that for comma +verb ing modifier to work following 2 conditions are to be met :
1) the comma + verb ing modifier must modify the previous clause
2) the comma +verb ing modifier must make sense with the subject of the previous clause

now my question is is it always possible to satisfy condition no #2
i am asking this because at times i have found that condition no #2 does get violated
for instance consider the following sentence : FIVE FLEDGLING SEA EAGLES left their nests in western Scotland this summer, BRINGING to 34 the number of wild birds successfully raised since transplants from Norway began in 1975

in this sentence the subject "FIVE FLEDGLING SEA EAGLES" does not make sense with the verb ING modifier .i do agree that the comma + verb ing modifier does modify the previous clause though .

Rule 2: The agent of the VERBing modifier should be the preceding subject (or the nearest preceding implied subject).
SC47 in the OG12 seems to abide by this rule.
When the eagles LEFT their nests, they were -- as a result of this action -- BRINGING to 34 the number of wild birds successfully raised.
In leaving their nests, the eagles showed the world that they had been successfully raised.

so can i assume that at times i might find sentences in which condition #2 might get violated?

also plz tell me that if my above analysis is correct then is this sentence correct :Crime has recently decreased in our neighborhood, leading to a rise in property values

While this sentence would be fine in the real world, I'd be skeptical if it appeared on the GMAT.
In every example that I posted above, the agent of the COMMA + VERBing modifier is the preceding subject (or the nearest preceding implied subject).
Here, the agent of the COMMA + VERBing modifier seems to be crime (the nearest preceding subject).
The conveyed meaning -- that crime was leading to a rise in property values -- is strange.
The agent of the VERBing action is not crime itself but the DECREASE in crime.
The following seems more GMAT-friendly:
A decrease in neighborhood crime has buoyed homebuyers, leading to a rise in property values.
In the revised sentence, the conveyed meaning is that a DECREASE in crime has buoyed homebuyers, and -- as a result of this action -- the decrease is leading to a rise in property values.