求助,xdf-s3-25, to do v.s. doing

isuffering

25. To speak habitually of the “truly needy” is gradually instilling the notion that many of those who are just called “needy” actually have adequate resources; such a conclusion is unwarranted.
(A) To speak habitually of the “truly needy” is gradually instilling the notion
(B) To speak habitually of the “truly needy” is instilling the notion gradually
(C) To speak habitually of the “truly needy” is gradually to instill the notion
(D) Speaking habitually of the “truly needy” is to instill the gradual notion
(E) Speaking habitually of the “truly needy” is instilling the gradual notion

I know the answer should be C, considering   "format consistency" and the "position of the word 'graudaully'".

But I'm not so sure the difference between " doing sth."and "to do sth.", I think both are Ok in this question.

I want to know the Gammar rule of "doing sth. v.s to do sth." in Gmat.

Can anybody help answer systematiclly?

Thanks in advance.

tongxun

-ing may indicate something has been completed or as a result.
to do... may denote something has not been finished yet or as a wish

isuffering

thanks, tongxun, good point.