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
Aerial carbon dioxide level 
Select and explain. 
A. 40 mm Hg, B. 37 mm Hg, C. 45 mm Hg, D. 60 mm Hg
A: 40 mm Hg
Answer: A: 40 mm Hg 
Explanation: A i.e. 40 mmHg