The parathyroid glands respond rapidly to hypocalcemia.The protein responsible for the rapid response of the principal cells to a falling calcium level is:

A)The PTH receptor.
B)Membrane Ca⁺⁺-adenosine triphosphatase (ATPase).
C)Ca⁺⁺-sensing receptor.
D)The Na⁺-Ca⁺⁺ exchange antiporter.
E)The transient receptor potential cation channel subfamily V,member 5 (TRPV5)calcium channel.