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 and should remain
(C) tells individual genes both when to become active and when to remain
(D) tells individual genes both when to activate and when to
(E) tell an individual gene both about when it should become active and when it should remain