The substance which is completely reabsorbed from the filtrate in the renal tubule under normal conditions is

The amount of the original filtrate volume which is reabsorbed by the time the tubular fluid enters the Henle's loop is approximately (in $\%$)

In the distal convoluted tubule $(DCT)$ of the nephrons:

Which of the following statements is correct regarding the reabsorption of $K^+$ (Potassium) in the nephron?

Which process is responsible for the excretion of drugs and toxins from the blood into the filtrate?

$A :$ Tubular secretion is of considerable importance in marine teleost fishes.
$R :$ These have aglomerular kidney,$i.e.$,no filtration occurs and tubular secretion is the only way of excretion.