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
Commonly used radioisotope for measuring red cell volume 
Select and explain. 
A. Chromium, B. Cobalt, C. Iron, D. Hydrogen
A: Chromium
Answer: A: Chromium 
Explanation: Red cell volume is commonly measured by tagging RBC with Chromium 51.