When left alone, certain specialized cardiac muscle cells have the capacity for self-excitation. The SA node is a collection of such cells. The SA node is innervated by the vagus nerve. The frequency of self excitation of the cardiac cells of the SA Node is likely to be:

A.  slower than a normal heartbeat because excitation by the vagus nerve decreases the heart rate.

B.  slower than a normal heartbeat because excitation by the vagus nerve increases the heart rate.

C.  faster than a normal heartbeat because excitation by the vagus nerve decreases the heart rate.

D.  faster than a normal heartbeat because excitation by the vagus nerve increases the heart rate.

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