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
D20 (Deuterium oxide) is used to measure volume of 
Select and explain. 
A. Blood, B. Total body water, C. Extracellular fluid, D. Intracellular fluid
B: Total body water
Answer: B: Total body water 
Explanation: B i.e. Total body water