It from Verbal SC 27:

27. Obtaining an investment-grade rating will keep the county's future borrowing costs low, protect its already-tattered image, and increase its ability to buy bond insurance.

(A) Obtaining an investment-grade rating will keep the county's future borrowing costs low, protect
(B) To obtain an investment-grade rating will keep the county's future borrowing costs low, and protect
(C) Having obtained an investment-grade rating will, in keeping the county's future borrowing costs low, protect
(D) To obtain an investment-grade rating would keep the county's future borrowing costs low, protecting
(E) Obtaining an investment-grade rating, keeping the county's borrowing costs low, would be protecting

OA: A

the explanation of D: The faulty parallelism is confusing; PROTECTING introduces an adjective phrase describing the subject of the sentence, but this makes INCREASE into a verb with no subject.

I don't understand 'this makes increse into a verb with no subject'. Does it because the protecting makes the phrase become a modifier thus interrupt the parallel?

Please help me, thanks so much!