The $SA$ node is located in

The post-caval vein collects blood from

Which artery carries deoxygenated blood instead of oxygenated blood?

What is correct about the sinus venosus?

What is the significance of the time gap in the passage of the action potential from the sino-atrial node to the ventricle?

$A$: Endocardium provides a smooth lining for the inside of the heart and covers the valves of the heart.
$R$: Endocardium is continuous with the endothelial lining of the large blood vessels associated with the heart and the rest of the cardiovascular system.