Which of the following is a secondary pollutant?

Give differences: $BOD$ and $COD$.

Sound becomes a hazardous noise pollution if its level exceeds (in $, dB$)

If $O_2$ is consumed more by microorganisms in water,what does it indicate?

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.

In $1984$,the Bhopal gas tragedy was caused due to the leakage of