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
The oxygen tension of the mixed venous blood is: 
Select and explain. 
A. 25 mm Hg, B. 40 mm Hg, C. 55 mm Hg, D. 70 mm Hg
B: 40 mm Hg
Answer: B: 40 mm Hg 
Explanation: None