Lowercase b is the allele that goes in 3
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.
I Think The answer is a or c Try Those options if There wrong I’ll fix Them for you To be sure you get The right answer Message Me if I’m wrong
