How Is It Possible to Have Two Separate Circulatory Systems

Multiple Choice

How is it possible to have two separate circulatory systems (pulmonary and systemic) ?

A) The pulmonary arteries and veins reverse their function at the heart so that the pulmonary veins carry oxygenated blood and the pulmonary arteries carry deoxygenated blood.
B) The heart is really two separate pumps; circulation to the lungs needs one pump, whereas circulation to the body needs a separate pump.
C) The two systems are separate to prevent mixing of oxygenated and deoxygenated blood, which would cause the oxygen level to drop too low.
D) The two systems are not truly separate; circulation of a specific sample of blood to the lungs occurs only after it has traveled through the body. It is one continuous system.