Child prodigies

[crackgmat007]

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 Pls explain

[cata1yst]

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 Pls explain

[leaonae]

The question tests the idiom 'not so much .. as".

Only one option, (D) uses the correct idiom. Others are don't sound good to me.