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
CORRECTC. 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
decongestingTwo 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.