Concept:Double circulation means blood travels through the heart twice in one complete circuit of the body.
Explanation:In mammals, blood first passes from the heart to the lungs and back to the heart.
This is the pulmonary circulation.
Then the blood is pumped from the heart to the rest of the body and returns again to the heart.
This is the systemic circulation.
Thus, in one full circuit, blood enters the heart twice: first for oxygenation in the lungs, then for distribution to body tissues.
The correct description is that blood passes through the heart twice in a complete circuit.
Answer:A. It passes twice through the heart in a complete circuit.