Answer : Option C
Explanation :
In moist conditions, sulphur dioxide is a bleaching agent. In the reducing action of SO2, oxygen is removed from coloured dye. Sulphur dioxide is used to bleach wool and silk. Salts of sulphur dioxide are also used in bleaching. Calcium hydrogen sulphite Ca(HSO3)2 is used to whiten wood pulp in the manufacturing of paper. Other than SO2, sodium hydrosulphite (Na2S2O4) is also used to bleach wool. However, bleaching using SO2 is not permanent. On exposure to air and light, the original colour slowly returns as oxygen in air oxidises the bleached substance.