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: Biochemistry
[{'content': 'The concentration of sodium ion in extracellular fluid is ___. \nSelect and explain. \nA. 10 mmol/L, B. 140 mmol/L, C. 25 mmol/L, D. 100 mmol/L', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]