A new drug named ANTAG-CaSR has been developed that is an antagonist at calcium-binding sites of CaSRs (Calcium-Sensing Receptors) in the plasma membranes of parathyroid gland cells. 

Healthy Person P receives regular doses of ANTAG-CaSR as part of a clinical trial.  When ANTAG-CaSR levels in the extracellular spaces surrounding parathyroid gland cells increase in Healthy Person P, this leads to

A. an increase in the levels of calcium in the blood plasma.

B. a decrease in the levels of parathyroid hormone (PTH) in the blood plasma.

C. an increase in the amount of PTH binding to PTH Receptors in bone.