The Concentration of the Primary Messenger (Ligand)in the Bloodstream Changes

The concentration of the primary messenger (ligand) in the bloodstream changes from <10⁻⁹ M when the signal is not being secreted to 10⁻⁷ M when it is being secreted.Which of the following K_ds for binding that ligand would be optimal for a receptor involved in a responsive signaling system?

A) 10⁻¹⁰
B) 10⁻⁹
C) 10⁻⁸
D) 10⁻⁷
E) 10⁻⁶