The atria and ventricles work together, alternately contracting and relaxing to pump blood through your heart. ... The electrical activity spreads through the walls of the atria and causes them to contract. This forces blood into the ventricles. The SA node sets the rate and rhythm of your heartbeat.
I would pick A hope it’s correct
In bone growth, the medullary cavity is enlarged because of the activity of osteoclasts. These are cells working to break down bone, resorbing old bones which will line in the medullary cavity. The cavity is also enlarge as the cells contribute to the increase in length and diameter of a growing bone.
