Which of the following diagrams is correct with regard to the proximal $(P)$ and distal $(D)$ tubule of the Nephron?

The given figure shows the human excretory system with parts labeled $A$ to $D$. Select the option that correctly identifies these parts and describes their characteristics or functions.

Complete the following:
$(a)$ Urinary excretion = tubular reabsorption + tubular secretion -..........
$(b)$ Dialysis fluid = Plasma -..........

Find the incorrect option regarding the mechanism of urine formation in humans.

The primary function of the kidney is ........ .

Which of the following statements is/are true?
$I.$ Urine is hypertonic while entering distal convoluted tubule.
$II.$ When the urine moves in the collecting tubule,it becomes hypertonic.
$III.$ Urine is isotonic in proximal convoluted tubule.
$IV.$ Urine becomes more and more hypotonic as it passes through the descending Henle's loop.