The rapid-rail lines that were recently introduced in

B for me.

A and C are out, as idiom "not only..but also" is not followed. E is awkward. Left with B and D. I will go with B, as "the lines" in B clearly refer to the lines mentioned previously in the sentence, whereas in D it is just "lines", which can be ambiguous.

OA??

Can any one explain how is
not only lowered pollution levels parallel to but also helped to decongest

hey sudhir
though its quite weird to help you (as you had 0 incorrect in MGMAT)

but still here is my view on this
not only lowered .......xxxxxx || but also helped ..............xxxxx

now xxxxx could be anything
we need not to consider its role in the sentence for us lowered should be "parallel" to "helped"
forget what is ahead in the sentence

I choose B.
My explanations are below

Can you explain howz that the correct idiom?
Not only X but also Y., both X and Y must be parallel .. are they?
I dont think so they are in B.

I think they are.

the lines have not only lowered something, but also helped in something

Crick,

This is a straight B.

'To help' is a special verb form that can be followed either by a bare infinitive or by a to-form infinitive. It is rarely used in the form Verb + Preposition + gerund. There are very few verbs that are followed by a preposition and a gerund. GMAT almost always prefers the to-form infinitives over the gerund-form.

This is the order GMAT usually prefers in order of correctness:
I helped to raise angel capital >> I helped raise angel capital >> I helped in raising angel capital

Hence, if "helped to decongest" were replaced "with" "helped in decongesting", it would be an incorrect answer choice.

Also a small thing (unrelated to question), the correct idiom usage of replace is 'to replace with' and not 'to replace by'.

Hope it helps.

B has correct idiom usage and also its a stand alone sentence unlike D which misses "The" infront of lines.

Thanks a lot for the explanation !! +1 Kudos !!

Crick