State whether the following statements are true or false:
$(1)$ The concentration of oxygen in cold water is $100 \ ppm$.
$(2)$ If the concentration of dissolved oxygen in water is less than $6 \ ppm$,the growth of fish gets inhibited.

Which air pollutant is responsible for the Bhopal gas tragedy?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R:$
Assertion $A:$ Polluted water may have a value of $BOD$ of the order of $17 \, ppm$.
Reason $R:$ $BOD$ is a measure of oxygen required to oxidise both the biodegradable and non-biodegradable organic material in water.
In the light of the above statements,choose the most appropriate answer from the options given below.

The effect of smog is observed on which of the following?

The layer of atmosphere between $10 \, km$ to $50 \, km$ above the sea level is called as

Identify $X$,$Y$,and $Z$ in the following reaction. (Equation not balanced)
$ClO + NO_2$ $\rightarrow \underline{X}$ $\xrightarrow{H_2O} \underline{Y} + \underline{Z}$