179. Biologists believe that they have found one of the substances that tell individual genes both when to become active and when to remain quiescent in the earliest phases of an embryo’s development.

(A) tell individual genes both when to become active and when to remain
(B) tell individual genes both at which time they should become active or should remain
(C) tells individual genes both when to become active or remain
(D) tells individual genes both when to become active or when to remain
(E) will tell an individual gene both about when it should become active and remain

Can you specify the answer pls.

my thoughts -
A) 'one of the substances' is singular - so 'that tell' is wrong
B) same as A
C) corrects 'that tells' But ignores parallelism - both X and Y
D) corrects 'that tells' and parallelism..my bet
E) changes tense

thoughts?? corrections?