Which of the following intensity of sound becomes a pollutant (in $\text{ dB}$)?

Pollution brings about undesirable changes in the $..........$ characteristics of air,land,or water.

Biochemical oxygen demand $(BOD)$ in a river water

Identify the correct pair:

In agriculture,improper drainage of water leads to:
$I-$ The soil becomes waterlogged.
$II -$ The soil surface draws up salts from the water.
$III -$ Salts are deposited as a thin crust on the land surface.
$IV -$ Salts start accumulating around the roots of the plants.
$V -$ The increasing concentration of salts becomes favorable for crop growth.
$VI -$ It is highly beneficial for agriculture.
Which of the above statements are correct?

The dental disease characterized by mottling of teeth due to the presence of a specific chemical in drinking water is caused by which of the following elements?