Answer:
D. The pacemaker.
Explanation:
The electrical signal starts in the sinoatrial (SA) node.
The SA node is also known as the heart’s natural pacemaker. It's located in the right atrium at the entrance of the superior vena cava.
The impulse starts in the sinus node (SA) then the electrical activity propagate and stimulate the myocardium, causing its contraction, this forces blood into the ventricles.
