Which of the following nitrogenous substances is highly toxic?

The most abundant metabolic waste product that is harmful and universal is ........ .

The characteristic odor of urine is due to the presence of . . . . . . .

The characteristic that is shared by urea,uric acid and ammonia is/are:
$(A)$ They are nitrogenous wastes.
$(B)$ They all need very large amount of water for excretion.
$(C)$ They all are equally toxic.
$(D)$ They are produced in the kidneys.

The characteristic that is shared by urea,uric acid and ammonia is/are:
$I.$ They are nitrogenous wastes.
$II.$ They all need very large amount of water for excretion.
$III.$ They are all equally toxic.
$IV.$ They are equally in the kidneys.

What is meant by the term osmoregulation?