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
[{'content': 'The oxygen tension of the mixed venous blood is: \nSelect and explain. \nA. 25 mm Hg, B. 40 mm Hg, C. 55 mm Hg, D. 70 mm Hg', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]