Which of the Following Contribute to Maintaining the High Osmolarity

Multiple Choice

Which of the following contribute to maintaining the high osmolarity of the renal medulla?
I.active transport of salt from the upper region of the ascending limb
II.the spatial arrangement of juxtamedullary nephrons
III.diffusion of urea from the collecting duct
IV.diffusion of salt from the descending limb of the loop of Henle

A) All of these conditions contribute to the osmolarity of the medulla.
B) I, II, and III
C) I and IV
D) I, II, and IV