Which of the following serves as an effector, or part of an effector, that functions in a negative feedback system?
A. 1,25-dihydroxyvitamin D Receptors located intracellularly in Parathryroid Gland cells.
B. Parathryroid Hormone Receptors (PTHRs) located in the plasma membranes of Parathyroid Gland cells.
C. CaSRs (Calcium-Sensing Receptors) in the plasma membranes of Parathryroid Gland cells.