Dissociation Constant Refers to Which of the Following Descriptions

Dissociation constant refers to which of the following descriptions?

A) Determines the portion of administered anesthetic dose in the lipid-soluble state (RN)
B) Determines the onset of anesthetic action as the increase of molecules crossing the nerve membrane decreases the time of the anesthetic's onset
C) The pH at which 50% of the molecules exist in the lipid-soluble form and 50% in the water-soluble form
D) All of the above