Which of these vessels returns blood to the left atrium of the heart?
a) pulmonary veins
b) coronary sinus
c) superior vena cava
d) pulmonary trunk

A

Explanation:

Pulmonary vein carries oxygenated blood from the lungs to the left atrium of the heart. Before this, the heart pumps deoxygenated blood to the lungs through the pulmonary artery. In the lungs, this blood expels carbon IV oxide and takes up oxygen. Haemoglobin in the Red Blood Cells is responsible for this. In the heart, this oxygenated blood is pumped by the left ventricle to the rest of the body through the aorta artery.  This oxygen is significant in cellular respiration in tissues.