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
Which of the following is the range of hemoglobin O2 saturation from systemic venous to systemic aerial blood represents a normal resting condition? 
Select and explain. 
A. 25 to 75%, B. 40 to 75%, C. 40 to 95%, D. 75 to 98%
D: 75 to 98%
Answer: D: 75 to 98% 
Explanation: 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.