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': 'D20 (Deuterium oxide) is used to measure volume of \nSelect and explain. \nA. Blood, B. Total body water, C. Extracellular fluid, D. Intracellular fluid', 'role': 'user'}, {'content': 'Answer: B \nExplanation: B i.e. Total body water', 'role': 'assistant'}]