In a polluted lake,the index of pollution is

The following water samples are given with their $BOD$ values. Which sample has the lowest level of pollution?
Sample $w - 6 \ mg/L$,Sample $x - 6.5 \ mg/L$
Sample $y - 7 \ mg/L$,Sample $z - 10.3 \ mg/L$

Domestic waste primarily contains:

The major drawback of $DDT$ as a pesticide is that:

What are the three major kinds of impurities in domestic wastewater?

When domestic sewage mixes with river water,