Concept:Double circulation in mammals means blood passes through the heart twice during one complete journey around the body.
Explanation:In mammals, the circulatory system consists of two separate loops.
The pulmonary loop carries deoxygenated blood from the heart to the lungs, where it picks up oxygen, and then returns it to the heart.
The systemic loop then pumps this oxygenated blood from the heart to all other body organs and brings it back again.
Because the blood enters the heart twice for every single complete circuit, the circulation is described as double.
This arrangement keeps oxygenated and deoxygenated blood fully separated, which improves the efficiency of oxygen delivery to the tissues.
The presence of arteries, veins, auricles, or ventricles does not explain why the circulation is called double.
Answer:Option A — blood passes through the heart twice in a complete circuit.