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The rapid-rail lines that were recently introduced in southern California have had positive effects: not only have they lowered pollution levels, they have helped in decongesting the region’s freeway system, which had been operating far above its capacity.

A.not only have they lowered pollution levels, they have helped in decongesting
Correct parallel marker is NOT ONLY X BUT ALSO Y. Hence Incorrect.
B. the lines have not only lowered pollution levels, but also helped to decongest
CORRECT
C. not only have pollution levels been lowered, but they have helped to decongest
Correct parallel marker is NOT ONLY X BUT ALSO Y. Hence Incorrect.
D. lines have lowered pollution levels, as well as helped in decongesting
Two results are not parallel. Hence Incorrect.
E. not only have they lowered pollution levels, but also have they helped decongest
Antecedent of THEY is ambiguous. hence Incorrect.
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Kritesh
The rapid-rail lines that were recently introduced in southern California have had positive effects: not only have they lowered pollution levels, they have helped in decongesting the region’s freeway system, which had been operating far above its capacity.

A.not only have they lowered pollution levels, they have helped in decongesting
B. the lines have not only lowered pollution levels, but also helped to decongest
C. not only have pollution levels been lowered, but they have helped to decongest
D. lines have lowered pollution levels, as well as helped in decongesting
E. not only have they lowered pollution levels, but also have they helped decongest

The correct idiom here is "not only x but y" and you could also have "not only x but also y." With this in mind we can eliminate A and D. E is just grammatically in correct. And the issue with C is the use of the pronoun "they." The way the sentence is written actually implies the pollution levels are what have helped to decongest the region's freeway system, which is clearly not correct.

B
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Kritesh
The rapid-rail lines that were recently introduced in southern California have had positive effects: not only have they lowered pollution levels, they have helped in decongesting the region’s freeway system, which had been operating far above its capacity.

A.not only have they lowered pollution levels, they have helped in decongesting [color=#ed1c24 no antecedent[/color]
B. the lines have not only lowered pollution levels, but also helped to decongest Correct
C. not only have pollution levels been lowered, but they have helped to decongest - this implies that the pollution levels have helped to decongest the region's freeway system
D. lines have lowered pollution levels, as well as helped in decongesting incorrect idiom
E. not only have they lowered pollution levels, but also have they helped decongest
no antecedent- the colon does not actually mean that you can refer back to the rapid-rail lines

The correct idiom is "not only X but Y"

B
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abhimahna
Not only X but also Y such that X and Y are parallel.

A.not only have they lowered pollution levels, they have helped in decongesting --> ||ism Error

B. the lines have not only lowered pollution levels, but also helped to decongest

C. not only have pollution levels been lowered, but they have helped to decongest --> ||ism Error

D. lines have lowered pollution levels, as well as helped in decongesting --> ||ism Error

E. not only have they lowered pollution levels, but also have they helped decongest --> TRAP. But they could refer to lines or positive effects. Ambiguity Error.


I don't understand how D has parallelism error.
"lines have lowered pollution levels, as well as helped in decongesting ..."

I understand that there is a noun after 'lowered' and a prepositional phrase after 'helped' but both 'lowered' and 'helped' are parallel.

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