The cardiac electrical impulse normally begins spontaneously in the sa node because
a. of the superior location of the sa node in the right atrium.
b. the sa node is the only area of the heart capable of spontaneous depolarization.
c. of the rich sympathetic innervation of the sa node.
d. this area depolarizes more rapidly than any other portion of the conduction system.
e. of action potentials from the cardioregulatory cente