Which observation best supports the statement that humans have double circulation of blood?

The correct answer is Blood passes through the heart twice in one complete circulation.

Double circulation is a mechanism of blood circulation in which blood flows through the heart twice during each complete cycle of the body. It consists of two separate pathways:
1. Pulmonary Circulation: The right side of the heart pumps deoxygenated blood to the lungs to pick up oxygen, and oxygenated blood returns to the left side of the heart.
2. Systemic Circulation: The left side of the heart pumps oxygenated blood to the rest of the body, and deoxygenated blood returns to the right side of the heart.

Because blood enters and exits the heart two separate times to complete one full journey throughout the body, humans are said to have double circulation. This mechanism prevents the mixing of oxygenated and deoxygenated blood, ensuring an efficient supply of oxygen to tissues.