This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
What is the paial pressure for carbon-di-oxide in the expired air? 
Select and explain. 
A. 0.3 mm Hg, B. 158 mm Hg, C. 40 mm Hg, D. 32 mm Hg
D: 32 mm Hg
Answer: D: 32 mm Hg 
Explanation: Paial pressure of CO2 in expired air (PE CO2) - 32 mm Hg Carbon-di-oxide: Paial pressure of CO2 in inspired air (Pi CO2) 0.3 mm Hg Paial pressure of CO2 in alveolar blood (PA CO2) 40 mm Hg Paial pressure of CO2 in expired air (PE CO2) 32 mm Hg.