For the best ecological balance,what percentage of the land mass of a country in the plains should be covered with forests (in $, \%$)?

Soil conservation can be best achieved by having

Contribution of individuals: Women of Garhwal/Himachal.

Arrange the following options in ascending order of their $BOD$ value.
$I.$ Sample of highly polluted pond water.
$II.$ Sample from unpolluted pond water.
$III.$ Distilled water.

Which one of the following is not a bioindicator of water pollution?

Identify the correct statements from the following:
$(i)$ $DDT$ is non-biodegradable.
$(ii)$ Heavy metals are responsible for biomagnification.
$(iii)$ High concentration of $DDT$ disturbs calcium metabolism in birds.
$(iv)$ The concentration of $DDT$ in large fish can be up to $2 \, ppm$.