The diagram depicts an alveolus and part of a blood vessel. The arrows indicate the direction in which gases move. Which arrows show oxygen only and carbon dioxide only?
- Aoxygen only: 1; carbon dioxide only: 2
- Boxygen only: 1; carbon dioxide only: 3
- Coxygen only: 2; carbon dioxide only: 3
- Doxygen only: 3; carbon dioxide only: 2