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 center

The correct answer will be option-D.

Explanation:

Conduction system of the heart contains autorhythmic fibres which are usually Nodes of the heart. There are two types of nodes-Sinoatrial node and the atrioventricular node.

The sinoatrial node is known as the pacemaker of the heart as it maintains the pace of the heartbeat due to its efficiency if higher depolarization than other node and conduction cells like a bundle of His and Purkinje fibres.  

Since depolarization causes a contraction in the muscle cells, therefore, heartbeats and thus option-D is the correct answer.

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