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: Medicine
Which of the following can be an approach in diagnosing polycythemia vera? 
Select and explain. 
A. Measure red cell mass, B. Measure serum EPO levels, C. Measure arterial oxygen saturation, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: None