What is used to reduce the emission of poisonous gases from vehicles?

What is $BOD$?

In a eutrophic water body,the excessive growth of $\underline {X}$ leads to the death of $\underline {Y}$.

Why do fish die in water bodies infested with water hyacinth ($Eichhornia$ $crassipes$)?

Most hazardous pollutant of automobile exhaust is

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?