Identify the pollutant gases largely responsible for the discoloured and lustreless nature of marble of the Taj Mahal.

A water sample has ppm level concentration of following anions
F^- = 10; $$SO_4^{2-}$$ = 100; $$NO_3^-$$ = 50
The anion/anions that make/makes the water sample unsuitable for drinking is/are :

Which one of the following substances used in dry cleaning is a better strategy to control environmental pollution?

BOD stands for :