$DDT$ is a

The $BOD$ limit prescribed by the Central Pollution Control Board $(CPCB)$ for the discharge of industrial and municipal wastewater into natural surface water bodies is .....

Effect of pollution is observed first on

Which pollutant undergoes biomagnification in a food chain?

Assertion $(A)$: Pollution is always caused by human activities.
Reason $(R)$: Pollution is not different from contamination.

In the context of sound levels,$dB$ stands for .........