Biological treatment of water pollution is done with the help of

In an area where $DDT$ had been used extensively,the population of birds declined significantly because

Assertion: Excess of nitrates in drinking water is harmful for infants.
Reason: Nitrates are responsible for blue baby syndrome.

What is pollution? Describe the causes of pollution.

Consider the following statements:
$I.$ Algal blooms are formed by free-floating algae.
$II.$ Algal blooms cause fish mortality and deterioration of water quality.
$III.$ Water hyacinth, the world's most problematic aquatic weed, is also called 'Terror of Bengal'.
Which of the statements given above are correct about algal blooms?

What is the major drawback of $DDT$ as an insecticide?