Final answer:
The correct answer is A. ribs. Ribs are not part of the respiratory system. The other options, B. trachea, C. diaphragm, and D. alveoli, are all elements of the respiratory system.
Step-by-step explanation:
The correct answer is A. ribs. Ribs are not part of the respiratory system. They are part of the skeletal system and provide protection to the organs in the thoracic cavity, including the lungs. The other options, B. trachea, C. diaphragm, and D. alveoli, are all elements of the respiratory system.