Green chemistry in day-to-day life is in the use of:

The statements that are $TRUE$:
$A$. Methane leads to both global warming and photochemical smog.
$B$. Methane is generated from paddy fields.
$C$. Methane is a stronger global warming gas than $CO_2$.
$D$. Methane is a part of reducing smog.
Choose the most appropriate answer from the options given below:

What is the percentage of oxygen by volume in dry air (in $\%$)?

Which gas was responsible for the Bhopal gas tragedy in $1984$?

Methane in polluted air reacts with ozone to form which of the following compounds?

$CF_xCl_y$ (where $x + y = 4$). These compounds are not used because