Explanation
The correct answer is B: SA Node.
The SA (sinoatrial) node, also known as the sinus node, is a group of specialized cells located in the right atrium of the heart. It is responsible for initiating the electrical impulses that start each heartbeat, and it sets the rate at which the heart beats. The impulses generated by the SA node travel through the atria, causing them to contract and push blood into the ventricles. The impulses then pass through the AV (atrioventricular) node, bundle of HIS, and Purkinje fibers, which coordinate the contraction of the ventricles and pump blood out of the heart. However, the SA node is the pacemaker of the heart and the main initiator of the heartbeat.
