The diagram shows a section of the human gas exchange system. Which row correctly identifies P, Q and R?
- AP: alveolus wall; Q: capillary wall; R: vein
- BP: bronchiole; Q: alveolus wall; R: capillary
- CP: capillary; Q: layer of moisture; R: bronchiole
- DP: alveolus wall; Q: bronchiole; R: capillary