What does a high $BOD$ of water indicate?

What happens when nitrates and phosphates are added to a lake?

Identify $X$ and $Y$ from the figure representing an Electrostatic Precipitator:
$X-Y$

$A$ disease caused by eating fish contaminated by industrial waste containing mercury compounds is known as

Which measures can be taken to control noise pollution?
$I-$ Use of sound-absorbent materials or mufflers in industries.
$II-$ Creating horn-free zones around hospitals and schools.
$III-$ Setting permissible sound levels for firecrackers and loud speakers.
$IV-$ Restricting the use of loudspeakers after a certain time.

Which is the most widely used insecticide in the world?