Biology help?

[schoolsux!]

What is the function of urea reabsorption in the collecting ducts of the kidney?
A) It restores the correct concentration of blood urea.
B) It increases the osmotic concentration of the interstitial fluid in the renal medulla so that more water can be extracted from the urine.
C) It reduces the salinity of the renal medulla and thus helps regulate how much water is reabsorbed from the urine.
D) It induces NaCl to move into the capillaries and thus helps maintain blood salinity.
E) The urea enters the ascending limb of the loop of Henle and from there is excreted in the urine.

In a dialysis machine, wastes are removed from blood plasma by a process of
A) filtration and reabsorption.
B) diffusion.
C) reabsorption and diffusion.
D) absorption.
E) absorption and reabsorption.