Which process occurs in the alveoli during external respiration?

• A. No gas exchange occurs in alveoli.
• B. Oxygen diffuses from alveoli to blood, and carbon dioxide diffuses from blood to alveoli.
• C. Oxygen and carbon dioxide both diffuse from alveoli to blood.
• D. Oxygen diffuses from blood to alveoli, and carbon dioxide diffuses from alveoli to blood.

Answer: (B)