This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Physiology
[{'content': 'Which of the following is the range of hemoglobin O2 saturation from systemic venous to systemic aerial blood represents a normal resting condition? \nSelect and explain. \nA. 25 to 75%, B. 40 to 75%, C. 40 to 95%, D. 75 to 98%', 'role': 'user'}, {'content': 'Answer: D \nExplanation: In a normal resting condition, the blood leaving the lungs is 98% saturated with oxygen, and the blood returning to the lungs is 75% saturated with oxygen. With vigorous exercise, blood leaving the lungs is still 98% saturated, but blood returning is usually less than 75% saturated because more oxygen is unloaded from hemoglobin in exercising muscles.', 'role': 'assistant'}]