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
The concentration of sodium ion in extracellular fluid is ___. 
Select and explain. 
A. 10 mmol/L, B. 140 mmol/L, C. 25 mmol/L, D. 100 mmol/L
B: 140 mmol/L
Answer: B: 140 mmol/L 
Explanation: None