In mammals,which valve guards the opening of the posterior vena cava into the right atrium?

In humans,blood passes from the post caval vein into the diastolic right atrium of the heart because of:

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:

'$Heart\; of\; Heart$' is

The splenic artery arises from the ..........

Match the following columns:

Column-$I$	Column-$II$
$a$. Superior Vena Cava	$p$. Carries deoxygenated blood towards the lungs
$b$. Inferior Vena Cava	$q$. Carries oxygenated blood away from the lungs
$c$. Pulmonary Artery	$r$. Brings deoxygenated blood from lower body parts to the right atrium
$d$. Pulmonary Vein	$s$. Brings deoxygenated blood from upper body parts to the right atrium