4. It is the process of breaking down food into simpler substances to be used by the body cells. A. Digestion B. Respiration C. Excretion D. Circulation 5. Which of the following breathing process is the movement of air into the lungs. A. Inhalation B. Respiration C. Exhalation D. Elimination 6.Which body system is responsible for breathing A. Digestive System C. Circulatory System B. Respiratory System D. Urinary System