Sentence Correction

Managing your irrigation system well and watering your grass wisely not only saves water, but helps to ensure the success of your lawn.

A. saves water, but helps to ensure the success of your lawn
B. saves water, but also help to ensure the success of your lawn
C. saves water, but the success of your lawn is more helpfully ensured
D. save water, but also help to ensure the success of your lawn
E. save water, but also the success of your lawn is more helpfully ensured

OAD
Source: GmatClub

Can some one explain me what is wrong with A?

X and Y Not only something but also something

X and Y imply compund subject -> plural
hence, saves (singular is out).
Down to D and E
D is concise,help is singular and save and help are parallel. hence D



Hope this helps.

My Pick - D,
1. Idiom: Not only.. but also [ A, C out ]
2. S-V agreement : Managing ... and watering... not only save ... but also help ..
3. meaning [E out]