Biology

The correct answer is C. Respiratory.

As you exercise, your circulatory system works in conjunction with the respiratory system to take in oxygen. The respiratory system is responsible for the exchange of gases, particularly oxygen and carbon dioxide, between the body and the environment.

During exercise, your body's oxygen demand increases to support the increased energy requirements of the muscles. The respiratory system facilitates this by inhaling oxygen-rich air into the lungs. Oxygen from the inhaled air diffuses into the bloodstream through the walls of the alveoli in the lungs.

Once in the bloodstream, the circulatory system, specifically the pulmonary circulation, transports oxygenated blood from the lungs to the heart. The heart then pumps this oxygenated blood to the rest of the body through the systemic circulation, delivering oxygen to the working muscles.

Option A, the muscular system, ...