The Mechanism for Maintaining the Solute Concentration Gradient in the Medulla

The mechanism for maintaining the solute concentration gradient in the medulla require(s)

A) active transport of Na⁺ ions from the ascending limb of the loop of Henle.
B) active transport of Na⁺ ions from the ascending limb of the Vasa Recta.
C) the ascending limb of the loop of Henle to be permeable to water.
D) the descending limb of the loop of Henle to be impermeable to water.
E) All of these are correct.