Sentence Correction

Question:

The final list of our sorority's designated charities - Children's Defense Fund, Coalition for Homeless, and American Humane Association - were arrived at only after months of painstaking research.

(A) were arrived at only after
(B) were arrived at on account of
(C) was arrived at by virtue of
(D) was arrived at only after
(E) was arrived at as a result of

I know that A and B are wrong.
I don't know why D is the correct answer.

Can anyone please explain this?

Thank you so much!

While in spoken English you could get away with C, D, or E, D is the best answer because the term "months" is a temporal term, hence "after" is the preposition that agrees with it.

C and E by virtue/as a result both point to the painstaking research itself, and ignore the temporal emphasis.