Sentence Correction

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

OA IS A

A) I understand that C is incorrect since it does not use the idiom " both X and y".
B) But then in answer choice A why should we use "tell" , "one of the substances is singular so cant we use "tells" ? of course answer choice A follows the idiom "both X and Y" and hence it is correct.


thanks