50. The high osmolarity of the renal medulla is maintained by all of the following except ________.
(A) diffusion of salt from the ascending limb of the loop of Henle
(B) active transport of salt from the upper region of the ascending limb
(C) the spatial arrangement of juxtamedullary nephrons
(D) diffusion of urea from the collecting duct
(E) diffusion of salt from the descending limb of the loop of Henle.