15. 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