Which of the following processes is $NOT$ used to preserve food?

Which of the following statements are correct?
$A$. Ocean is a sink for $CO_2$.
$B$. Greenhouse effect causes lowering of the temperature of the earth's surface.
$C$. To control $CO$ emission by automobiles,usually catalytic converters are fitted into exhaust pipes.
$D$. $H_2SO_4$,herbicides,and insecticides form mist.

What is the volume percentage of nitrogen gas in the atmosphere (in $\%$)?

Acrolein $(X)$ is one of the chemicals formed when $O_3$ and $NO_2$ react with unburnt hydrocarbons present in the polluted air. The structure of '$X$' is

The concentration of dissolved oxygen $(DO)$ in cold water can go up to ......... $ppm$.

The concentration of dissolved oxygen in water for the growth of fish should be more than $X \ ppm$ and the Biochemical Oxygen Demand $(BOD)$ in clean water should be less than $Y \ ppm$. $X$ and $Y$ in $ppm$ are,respectively: