Detergents and fertilizers containing phosphate compounds are responsible for

Substances that cause pollution or contaminate the environment are called .........

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.

Indiscriminate use of $DDT$ is undesirable because

Which pollutants are present in the troposphere?

$A$ water sample is considered the cleanest based on which of the following $BOD$ values (in $ppm$)?