Assertion: In the descending limb of loop of Henle the urine is hypotoonic, while in ascending limb of loop of Henle, the urine is hypertonic.
Reason: Descending limb is impermeable to water while ascending limb is impermeable to $N{a}^{+}$

Answer: DSolution: The descending limb of loop of henle is permeable to water but almost impermeable to electroytes. This concentrates the filtrate as it moves down. The ascending limb is impermeable to water but allows transport of electrolytes actively or passivly. Therefore, as the concentrated filtrate pass upward, it gets diluted due to the passage of electrolytes to the medullary fluid.