What is the lower region of the atmosphere that extends up to $10$ to $15 \ km$ from sea level called?

Write down the reactions involved during the formation of photochemical smog.

The presence of soluble fluoride ion up to $1 \ ppm$ concentration in drinking water is:

Match the following List-$I$ ($F^-$ ion concentration in drinking water) with List-$II$ (Effects on humans):

List-$I$	List-$II$
$(A)$ $< 1 \ ppm$	$(i)$ Harmful to bones
$(B)$ $> 2 \ ppm$	$(ii)$ Tooth decay
$(C)$ $> 10 \ ppm$	$(iii)$ Brown mottling of teeth

Then the correct match is:

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:

Chlorine gas reacts with the aromatic ring of lignin to form which of the following substances?