Answer:
The correct answer is D.
Step-by-step explanation:
The normal pacemaker of the heart is the sinoatrial node (SA). It is a small mass of specialized cells located at right atrium near the entrance of the superior vena cava.
If the SA node is fail to function, a group of cells from the atrioventricular node can take over the pacemaker function.
Also the Bundle of His and the Purkinje fibers can take over pacemaker function if none of the above function properly.