Which one of the following is not a common component of photo-chemical smog?

Polar stratospheric clouds were observed over Antarctica some time ago. How might they have formed,and what happens if some of these clouds break down due to heat in the presence of sunlight?

The structure of all living organisms on Earth is known as ............

Which is not a renewable source?

Classical smog is a mixture of .....

$A$ water sample has $ppm$ level concentration of the following metals: $Fe = 0.2$,$Mn = 0.05$,$Cu = 3.0$,$Zn = 5.0$. The metal that makes the water sample unsuitable for drinking is