$(a)$ Why does the Mathura refinery pose problems to the Taj Mahal?
$(b)$ Why do lichens not occur in Delhi,whereas they commonly grow in Manali or Darjeeling?
$(c)$ What is smog? Mention its harmful effects.

(N/A) The Mathura refinery releases oxides of nitrogen and sulphur into the atmosphere. These gases react with water vapor to form acid rain,which causes the corrosion of the marble of the Taj Mahal.
$(b)$ Lichens are highly sensitive to sulphur dioxide $(SO_2)$ pollution. Delhi has high levels of air pollution due to industrial activities and vehicular emissions,which prevents the growth of lichens. In contrast,places like Manali and Darjeeling have cleaner,humid environments that are suitable for lichen growth.
$(c)$ Smog is a combination of smoke and fog particles in the atmosphere.
Harmful effects of smog:
$(i)$ It significantly reduces visibility,which disrupts road and air traffic.
$(ii)$ It causes severe respiratory and breathing problems in humans.