Why is it advised that industries should switch over to cleaner fuels such as CNG and LPG in the Taj Mahal zone in Agra?

Why is it advised that industries should switch over to cleaner fuels such as CNG and LPG in the Taj Mahal zone in Agra?
Answer:
The Taj Mahal, India’s most famous tourist attraction is getting discoloured because of air pollutants. Industries like rubber processing, automobile and chemical industries produce pollutants like sulphur dioxide and nitrogen dioxide that react with water vapour present in the air to form sulphuric add and nitric acid. These mix with rain and fall down as acid rain. This acid rain corrodes the marble of the Taj Mahal. This phenomenon is also referred to as ‘marble cancer’. So, industries are advised to use cleaner fuels like CNG (Compressed Natural Gas), LPG (Liquefied Petroleum Gas), etc. to save the Taj Mahal.