What is the symbolic representation $dB$ used for?

Which of the following is a weightless pollutant?

Eutrophication results in the reduction of

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.

Prime contaminants leading to cultural or accelerated eutrophication are

Which of the following metals is a water pollutant and causes sterility in human beings?