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': 'Commonly used radioisotope for measuring red cell volume \nSelect and explain. \nA. Chromium, B. Cobalt, C. Iron, D. Hydrogen', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Red cell volume is commonly measured by tagging RBC with Chromium 51.', 'role': 'assistant'}]