<span>The best definition of a population is </span><span>D. The total number of individuals in a given area. Keyword "Individual." and "Total number".</span>
It is about 78%, I hope that this helped :)
Since the other answer got deleted, i'll just answer it for you.
a sinoatrial node is the main pacemaker that begins each heartbeat located in the junctions of superior vena cava and the right atrium. It signals from the node that spread through the atria and determines the heart rates.