Which of the following would help maintain the medullary concentration gradient?
A. movement of urea from the distal tubule to the proximal tubule
B. urea movement from the collecting duct to medullary interstitial fluid
C. movement of water from the ascending limb into the medullary interstitial fluid
D. active transport of sodium and chloride from the medullary interstitial fluid into the collecting duct
E. urea movement into the collecting duct
