Why Must Blood Follow a Double Circuit (Pass Through the Heart

Why must blood follow a double circuit (pass through the heart twice before moving out to the body) ?

A) Blood must be oxygenated in the pulmonary circuit prior to moving to the systemic circuit where it moves out to the body.
B) Blood must pick up nutrients in the gut prior to entering the heart a second time to be oxygenated in the pulmonary circuit.
C) Blood does not move through the heart twice before moving out to the body.
D) Blood must go to organs with high oxygen demands prior to moving back to the lungs in the pulmonary circuit.
E) Both the pulmonary and systemic circuits bypass the heart,which acts only to provide pressure to move the two systems.