Final answer:
When the etCO2 (end-tidal carbon dioxide) level is 5%, it is equivalent to 40 mmHg.
Step-by-step explanation:
The partial pressure of carbon dioxide in the blood is 45 mmHg, while the partial pressure in the alveoli is 40 mmHg. The difference in partial pressure causes carbon dioxide to diffuse from the blood into the alveoli. Therefore, when the etCO2 (end-tidal carbon dioxide) level is 5%, it is equivalent to 40 mmHg. So, the answer is b) 20 mmHg.