I think the answer is C because of the use of will[/u]

RBBmba@2014 wrote:Because the population is denser there and the automobile is therefore inefficient as a means of transportation, a commuter rail system serving a given population is usually five times more efficient in a European city than it is in an American city.

a) serving a given population is usually five times more efficient in a European city than it is in an American city

@ Mitch - confused with this part of option A.

Why "it " can't refer to the 'commuter rail system ' ONLY instead of referring to 'commuter rail system serving a given population' as a whole ?

P.S: PLEASE NOTE that I don't have any concern in understanding why A is incorrect (re the antecedent issue of the introductory modifier of 'Because-clause') , just would like to clarify the above aspect.

A RESTRICTIVE MODIFIER serves to RESTRICT OR DEFINE the noun it modifies.
The man standing on the corner is tall.
Here, standing on the corner is a restrictive modifier.
The subject of the sentence is not a man in general but a SPECIFIC MAN:
the man STANDING ON THE CORNER.

When the antecedent for a subject pronoun (he, she, it, they) is accompanied by RESTRICTIVE MODIFIERS, the subject pronoun serves to refer to the ANTECEDENT + ALL RESTRICTIVE MODIFIERS.
This is simply how subject pronouns work.
A: A commuter rail system serving a given population is usually five times more efficient in a European city than it is in an American city.
Here, serving a given population is a restrictive modifier.
As a result, it serves to refer not just to a commuter rail system but to the ENTIRE PHRASE IN RED.

GMATGuruNY wrote: In my first post above, I offered the easiest way to eliminate C.

Another reason is that C implies the following comparison:
A commuter rail system WILL USUALLY BE FIVE TIMES MORE EFFICIENT IN A EUROPEAN CITY than one serving a comparable population in an American city [WILL BE EFFICIENT IN A EUROPEAN CITY].
The words in brackets are omitted, but their presence is implied.
Not the intended meaning.

Hi GMATGuruNY,

(1) could you please clarify whether the following is a valid ERROR in C ?

In C, A commuter rail system WILL USUALLY BE is compared to one serving - this is INCORRECT, I think, because, SUBJECT + VERB is NOT GRAMMATICALLY PARALLEL to PRONOUN + (VERBing) MODIFIER. Right ?

(2) However, if C were "A commuter rail system will usually be five times more efficient in a European city than one (=a commuter rail system) serving a comparable population in an American city (WILL BE)" , then would C be correct (as far as COMPARISON is concerned in GMAT) ?

GMATGuruNY wrote: Incorrect: The books on the bottom shelf are less expensive than on the top shelf.
Here -- because the second clause lacks an explicit subject -- both clauses seem to be about the books on the bottom shelf.
The result is a nonsensical comparison:
The books on the bottom shelf are less expensive than [the books on the bottom shelf] on the top shelf [are expensive].
To make it clear that the second clause is about a DIFFERENT set of books, the second clause requires ITS OWN SUBJECT:
The books on the bottom shelf are less expensive than THOSE on the top shelf.
Conveyed meaning:
The books on the bottom shelf are less expensive than those on the top shelf [are expensive].

A quick question on this - why it'd be wrong to consider the (DROPPED) SUBJECT in the SECOND CLAUSE is ONLY the BOOKS, but NOT the books on the bottom shelf ?

As for the OA: in a European city will usually be five times more efficient than will [BE] a system serving a comparable population in an American city -- I think, the RED part is dropped as ELLIPSIS. Right ?

Hi GMATGuruNY - could you please share your feedback on my above raised doubts ?

Look forward to your explanations. Much thanks in advance!

Hi GMATGuruNY - any update on my above concerns Sir ?

It'd be really helpful to know your explanations. Much thanks in advance!

RBBmba@2014 wrote:

GMATGuruNY wrote: In my first post above, I offered the easiest way to eliminate C.

Another reason is that C implies the following comparison:
A commuter rail system WILL USUALLY BE FIVE TIMES MORE EFFICIENT IN A EUROPEAN CITY than one serving a comparable population in an American city [WILL BE EFFICIENT IN A EUROPEAN CITY].
The words in brackets are omitted, but their presence is implied.
Not the intended meaning.

Hi GMATGuruNY,

(1) could you please clarify whether the following is a valid ERROR in C ?

In C, A commuter rail system WILL USUALLY BE is compared to one serving - this is INCORRECT, I think, because, SUBJECT + VERB is NOT GRAMMATICALLY PARALLEL to PRONOUN + (VERBing) MODIFIER. Right ?

(2) However, if C were "A commuter rail system will usually be five times more efficient in a European city than one (=a commuter rail system) serving a comparable population in an American city (WILL BE)" , then would C be correct (as far as COMPARISON is concerned in GMAT) ?

Your proposed revision of C:
C: A commuter rail system will be more efficient in a European city than one serving a given population in an American city will be.
In the first clause, in a European city is part of the PREDICATE (the portion of the clause that follows the verb and is NOT part of the subject).
In the second clause, in an American city is part of the SUBJECT.
The result is a lack of parallelism.

To maintain parallelism, the two modifiers about location should serve the SAME FUNCTION, as in the OA:
A commuter rail system in a European city will usually be five times more efficient than will a system serving a comparable population in an American city.
Here, in a European city is part of the subject of the first clause, and in an American city is part of the subject of the second clause.
The result is better parallelism.

GMATGuruNY wrote: Incorrect: The books on the bottom shelf are less expensive than on the top shelf.

A quick question on this - why it'd be wrong to consider the (DROPPED) SUBJECT in the SECOND CLAUSE is ONLY the BOOKS, but NOT the books on the bottom shelf ?

If the subject of the second clause is omitted, the implied subject of the second clause is the subject of the first clause and ANY ATTACHED MODIFIERS.
In the second clause above, the implied subject is the books on the bottom shelf (the preceding subject and its attached modifiers).

As for the OA: in a European city will usually be five times more efficient than will [BE] a system serving a comparable population in an American city -- I think, the RED part is dropped as ELLIPSIS. Right ?

Correct.

Good Day Guru

C: A commuter rail system will be more efficient in a European city than one serving a given population in an American city will be.

Guru i feel that there is one more thing WRONG in above quoted sentence: in this sentence the expression "in the european city" is ACTING as ADVERBIAL and thereby it is modifying "WILL BE MORE EFFICIENT" ---->there by implying that the FACT that this rail system is in european city is MAKING it more EFFICIENT

A copy pronoun such as those must serve to refer to both the preceding subject and ANY ATTACHED MODIFIERS.
Here, those must serve to refer to the books on the bottom shelf (the preceding subject and its attached modifiers).

Guru i think THOSE/ THAT are A NEW copy pronoun and refers to only the NOUN and NOT ITS ATTACHED MODIFIERS

thanks and best regards

aditya8062 wrote:Good Day Guru

C: A commuter rail system will be more efficient in a European city than one serving a given population in an American city will be.

Guru i feel that there is one more thing WRONG in above quoted sentence: in this sentence the expression "in the european city" is ACTING as ADVERBIAL and thereby it is modifying "WILL BE MORE EFFICIENT" ---->there by implying that the FACT that this rail system is in european city is MAKING it more EFFICIENT

Correct.
In C, in a European city seems to be an ADVERB modifying efficient, expressing WHERE the system is EFFICIENT.
Not the intended meaning.
The intended meaning is to express where the system is LOCATED, as in the OA:
a commuter rail system in a European city.

Guru i think THOSE/ THAT are A NEW copy pronoun and refers to only the NOUN and NOT ITS ATTACHED MODIFIERS

thanks and best regards

Good catch.
My response above was intended to read as follows:
If the subject of the second clause is omitted, the implied subject of the second clause is the subject of the first clause and ANY ATTACHED MODIFIERS.
I've amended my post accordingly.

Thanks Mitch.

Just couple of quick questions on your reply:

This construction in Option C -- SUBJECT + VERB in the FIRST clause and PRONOUN + (VERBing) MODIFIER in the SECOND clause -- is itself GRAMMATICALLY INCORRECT on GMAT because they're NOT PARALLEL to each other. Right ?

GMATGuruNY wrote:If the subject of the second clause is omitted, the implied subject of the second clause is the subject of the first clause and ANY ATTACHED MODIFIERS.
In the second clause above, the implied subject is the books on the bottom shelf (the preceding subject and its attached modifiers).

Is this ALWAYS TRUE on GMAT ?

Mitch - Would you please come up with your quick feedback on my above questions ?

Look forward to hear from you Sir! Much thanks in advance.

RBBmba@2014 wrote:Thanks Mitch.

Just couple of quick questions on your reply:

This construction in Option C -- SUBJECT + VERB in the FIRST clause and PRONOUN + (VERBing) MODIFIER in the SECOND clause -- is itself GRAMMATICALLY INCORRECT on GMAT because they're NOT PARALLEL to each other. Right ?

Generally, an error will result if the main VERB in the first clause is converted into a MODIFIER in the second clause.
An incorrect answer to an SC in GMATPrep:
Three times as many institutions charge tuition and fees of under $8,000 a year as those charging over $16,000.
Here, the main verb in the first clause (charge) is converted into a VERBing modifier in the second clause (charging).
The result is a lack of parallelism.

In C, the main verb in the first clause (will be) is NOT converted into a modifier in the second clause.
C would exhibit this error if it read as follows:
Rail systems in Europe typically serve more people than those serving people in the US.
Here, the main verb in the first clause (serve) is incorrectly converted into a VERBing modifier in the second clause (serving).

GMATGuruNY wrote:If the subject of the second clause is omitted, the implied subject of the second clause is the subject of the first clause and ANY ATTACHED MODIFIERS.
In the second clause above, the implied subject is the books on the bottom shelf (the preceding subject and its attached modifiers).

Is this ALWAYS TRUE on GMAT ?

Yes.