The marble of the Taj Mahal is threatened chiefly by which of the following pollutants?

The main threat is sulphur dioxide. Emitted by the Mathura refinery, foundries and other industries around Agra, it forms sulphuric acid in moist air and acid rain, which reacts with the calcium carbonate of the marble and turns it into calcium sulphate, causing the pitting and corrosion called 'marble cancer' (the yellowing of the surface comes mainly from soot and dust particles). Carbon dioxide forms only the weak carbonic acid, carbon monoxide does not react with marble, and calcium oxide is not an air pollutant of the area.