The Osmotic Gradient in the Medullary Region of the Kidneys

The osmotic gradient in the medullary region of the kidneys is established and maintained by which of the following?

A) Na+/K+/Cl- cotransporters in the ascending limb of the loop of Henle
B) Na+/K+/Cl- cotransporters in the descending limb of the loop of Henle
C) efflux of Na+ from the descending limb of the loop of Henle
D) efflux of water from the descending limb of the loop of Henle
E) efflux of water from the ascending limb of the loop of Henle