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
Substance involved in countercurrent mechanism for maintaining medullary gradient - 
Select and explain. 
A. NaCl, B. Urea, C. NaCl, urea, D. NaCl, urea, water
D: NaCl, urea, water
Answer: D: NaCl, urea, water 
Explanation: D i.e. NaCI, Urea, Water