30. It has been demonstrated that thrombin binds to its receptor and induces a transient increase in the intracellular calcium level of vascular smooth muscle cells. After the intracellular calcium recovers to the basal level, the cells are no longer responsive to the 2nd challenges of thrombin. Which one of the following terms is the best description of this phenomenon?
(A) tolerance
(B) specificity
(C) tachyphylaxis
(D) withdrawal response