23. The dissociation constant (Ka) of the human serotonin receptor for serotonin is 1 x 10-10 M. What does this mean? (A) It means that when the concentration of the serotonin receptor is 1 x 10-10 M, half of the available serotonin receptor sites are occupied. (B) It means that when the concentration of the serotonin receptor is 1 x 10-10 M, all of the available serotonin receptor sites are occupied. (C) It means that when the concentration of serotonin is 1 x 10-10 M, all of the available serotonin receptor sites are occupied. (D) It means that when the concentration of serotonin is 1 x 10-10 M, half of the available serotonin receptor sites are occupied. (E) None of the above