This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Kidneys and Body Fluids, Subject name: Physiology
The prime driving force for counter current multiplier system is 
Select and explain. 
A. Medullary hyperosmolarity, B. Reabsorption of Na+ in thick ascending limb, C. Action of ADH via aquaporin channels, D. Urea recycling
B: Reabsorption of Na+ in thick ascending limb
Answer: B: Reabsorption of Na+ in thick ascending limb 
Explanation: Ans. (b) Reabsorption of Na+ in thick ascending limb(Ref: Ganongy 25th ed/p.685)The primary driving force for counter current system is Reabsorption of Na+ in thick ascending limb through Na-K-2Cl cotransporter