During exercise, sensory receptors detect changes in oxygen and carbon dioxide levels in the blood. This information is then sent to the brain's medulla, which causes the heart to beat faster and increases the rate of breathing. Exercising also causes the nervous system to signal the pancreas and thyroid gland to release hormones which regulate metabolism. Which statement is best demonstrated by this scenario? A. Hormones act as signal molecules between all body systems. B. Increased heart rate and blood flow decrease oxygen supply. C. Exercise is important for the body to function properly. D. Nerves transmit and respond to every major system in the body.