How many sentences are correct?
$(1)$ $CNG$ burns most efficiently.
$(2)$ $CNG$ cannot be adulterated.
$(3)$ $CNG$ is cheaper than petrol and diesel.
$(4)$ In the $1990$s,Delhi ranked $4$th among the $41$ most polluted cities of the world.

The figure shows the biomagnification of $DDT$ in an aquatic food chain. What could be the concentration of $DDT$ in $P$,$Q$,and $R$ respectively?

More than $70\%$ of the world's freshwater is contained in $............$.

Consider the following statements:
$I.$ Reforestation is the process of restoring a forest that once existed but was removed at some point of time in the past.
$II.$ Reforestation may occur naturally in a deforested area.
$III.$ $A$ tree plantation movement or Van Mahotsava is being carried out in India since $1982$.
Which of the statements given above are correct?

What is the full form of $NGO$?

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 $2 \, ppm$.