In a polluted lake:

In the treatment of wastewater,scientists at the Arcata Marsh (a project associated with Humboldt State University) developed a series of six connected marshes over how much area to remove heavy metals and other pollutants?

Exposure to a noise level of $............$ or more for a short period can damage the eardrums,leading to permanent hearing impairment. (in $,dB$)

What is the significance of using unleaded petrol in motor vehicles with catalytic converter?

Give scientific reasons: The use of $CNG$ is more advantageous than the use of petrol.

What is the intensity of sound in normal conversation?