10. Blood returning to the mammalian heart in a pulmonary vein drains first into the
A. Left ventricle
B. Left atrium
C. Right atrium
D. Vena cava
E. Right ventricle

Option B.

Left atrium.

Explanation:

Pulmonary veins are veins that transmit oxygenated blood from the lungs to the heart. There are four largest pulmonary veins,two from the lungs that drain into the left atrium of the heart. The blood that is drained into the left atrium is pumped into the left ventricle through biscupid valve. The pulmonary vein function in respiration by receiving oxygenated blood in the alveoli and return it to left alveoli.