How Does the Loop of Henle Enable the Mammalian Kidney

Multiple Choice

How does the loop of Henle enable the mammalian kidney to produce hypertonic urine?
I-The ascending limb of the loop actively extrudes Na+.
II-The descending limb interacts with the ascending limb to raise the osmolarity of the extracellular fluid in the renal medulla.
III-The concentration is further enhanced by the presence of urea,which draws water out of the collecting ducts by osmosis.

A) just I
B) II and III
C) just III
D) I and II
E) I,II,and III