Henle's loop of nephron plays a significant role in maintaining a high osmolarity in

The human kidneys produce urine that is approximately .......... times more concentrated than the initial filtrate.

Which of the following statements about the reabsorption process in Henle's loop are correct?
$(a)$ The descending limb of Henle's loop is permeable to water but almost impermeable to electrolytes.
$(b)$ Urine gets concentrated in Henle's loop.
$(c)$ Reabsorption of $Na^+$ and water takes place in Henle's loop.
$(d)$ Active or passive transport of electrolytes occurs in the ascending limb of Henle's loop.
Choose the correct answer from the options given below:

The flow of $P$ in the limbs of the Loop of Henle and the flow of $Q$ in the limbs of the vasa recta create a counter-current mechanism.
$\quad P \quad\quad Q$

Select the correct option:
$(1)$ The counter-current mechanism is maintained by the Loop of Henle and the vasa recta.
$(2)$ The ascending limb of the Loop of Henle is impermeable to water.

The counter-current multiplier system changes the isotonic glomerular filtrate into