In a single cardiac contraction, or heartbeat, the entire heart contracts in series--first the atria and then the ventricles. What is the first step in this process?
a. An action potential is generated by the atrioventricular node.
b. An action potential is generated by the sinoatrial node.
c. An action potential is generated at the Purkinje fibers.
d. An action potential is generated at the Bundle of His



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