In the heart, as the action potential reaches the $AV$ node from the $SA$ node, there is a delay of the action potential. This delay is important because:

• A
  It allows atria to rest.
• B
  It allows a stronger right atrial contraction.
• C
  It allows ventricles to receive all the blood from the atria.
• D
  It allows right atria to receive the blood from vena cava.