[X] + H base 2SO base 4 ? [Y] a colourless gas with irri9tating smell, [Y] + K base Cr base 2O base 7 + H base 2SO base 4 ? green solution. [X] and [Y] are:

a. SO base 2^2-, SO base 2

b. CI^-,HCI

c. S^2-, H base 2S

d. CO base 3^2-,CO base 2

(a) SO base 2 and H base 2S, both being reducing agents, can turn acidified dichromater solution green. SO base 2 can be obtained by the action of acid upon sulphite, while H base 2S is evolved by the action of acid upon sulphide. However, SO base 2 has a burning sulphur smell which is irritating. H base 2S has rotten egg like smell.