Child prodigies

[phelps]

1. Child prodigies are marked not so much by their skills but instead by the fact that these skills are fully developed at a very early age.
(A) but instead
(B) rather than
(C) than
(D) as
(E) so much as


OA D

IMO B

Can anyone explain please

[Robinmrtha]

Not so much ...X as ... Y is correct idiom