<--[if !supportLists]-->178.<--[endif]-->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（A）D-22

tonyadidas:

one of 复数名词，that+动词复数

the one of 复数名词，that+动词单数

I got it, thanks !!!!