Cardiac muscle cells in the atria are capable of contracting spontaneously because of a spontaneous depolarization that results from:

A)An inward current through a hyperpolarization-activated channel.
B)Cyclic changes in the activity of Na⁺,K⁺-ATPase.
C)Slow leak of Ca⁺⁺ through the dihydropyridine receptor.
D)Cyclic changes in the activity of the sarcolemmal Ca⁺⁺ pump.
E)Cyclic changes in the activity of SERCA.