A study published in the journal Developmental Science hypothesized that when
babies are able to follow the gazes of adults and pay attention to their parents'
conversations, they not only display advanced social interaction skills upon
entering preschool but also developing language abilities more quickly
than do those unable to exhibit gaze-following and attentive behaviors.

A:...

B: they not only display advanced social interaction skills upon entering preschool
but also develop language abilities quicker than do

C: they not only display advanced social interaction skills upon entering preschool
but also develop language abilities more quickly than

D: not only do they display advanced social interaction skills upon entering
preschool but also develop language abilities quicker than

E: not only were they displaying advanced social interaction skills upon entering
preschool but also developing language abilities more quickly than

Between (B) and (C), which is the best response and why?
I use "more quickly" to modify "develop" and i arrived at C.

Hope am correct anyway, as I have no OA for the question.