A physiologist isolated a myocardial conducting cell and placed it in a tissue bath containing normal physiological solution, electrically stimulated the cell, and measured the action potential. The physiologist added amlodipine and saw prolongation of the rapid depolarization phase (phase 0) as well as a decrease in the amplitude of the action potential. He repeated the experiment with a myocardial contractile cell and saw an elimination of the plateau phase (phase 2) of the action potential. From these observations, which of the following was affected by amlodipine? a.) K⁺ leaving the cells b.) Rapid Na⁺ entry into the cells c.) Slow Na entry into the cells d.) Ca2⁺ entry into the cells