Which organ regulates the amount of carbon dioxide circulating in the blood?

Rationale: A. Kidneys regulate bicarbonate and acid-base balance but do not directly regulate CO₂ levels. B. Blood carries CO₂ but does not regulate its amount. C. Pancreas is involved in digestion and insulin secretion, not CO₂ regulation. D. Lungs regulate CO₂ by controlling its exhalation during respiration, directly affecting CO₂ levels in the blood.