Which of the following would help maintain the medullary concentration gradient?
A. Movement of water from the ascending limb into the medullary interstitial fluid
B. Movement of urea from the distal convoluted tubule to the proximal convoluted tubule
C. Urea movement into the collecting duct Active transport of Nat and CI- from the medullary interstitial fluid into the collecting duct
D. Urea movement from the collecting duct to medullary interstitial fluid