The partial pressures (in mm Hg) of oxygen ( O 2 ) and carbon dioxide ( CO 2 ) at alveoli (the site of diffusion) are:

1. A. pO 2 = 40 and pCO 2 = 45
2. B. pO 2 = 95 and pCO 2 = 40
3. C. pO 2 = 159 and pCO 2 = 0.3
4. D. pO 2 = 104 and pCO 2 = 40

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