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

State whether the following statements are true or false:
$(1)$ Severe respiratory diseases in children are caused by $NO_2$ gas.
$(2)$ Carbon monoxide binds with hemoglobin in the blood to form a carboxyhemoglobin complex.
$(3)$ The carboxyhemoglobin complex is $100$ times more stable than the oxygen-hemoglobin complex.
$(4)$ The $pH$ of rainwater is around $5.6$.

Assertion : Photochemical smog is produced by nitrogen oxides.
Reason : Vehicular pollution is a major source of nitrogen oxides.

Leakage of which gas was responsible for the Bhopal tragedy in $1984$?

The living organisms or the surroundings of the earth's composition are known as .....

How is biogas produced?