Answer: A
Explanation: It makes the most sense because it isn't directly speaking about something, it is mainly summarizing the main idea. I'm not 100%, but that's the one I would choose if it were me. Hope I helped out a little!
The main responsible for the delay among the atrial and
ventricular contractions is the action potential conduction velocity of the AV
node cells. If the atria and ventricles contract simultaneously, atria cannot
give any help in filling the ventricles, which is needed, especially in those
with stiff ventricles as in the elderly. Hence atria have to contract first and
then the ventricles.
