Amongst the various ores of a metal (M) (sulphide carbonates, oxides, hydrated or hydroxides) two ores [X] and [Y] show the following reactivity. 

(a) [X] on calcination gives a black solid (S), water and a colourless gas which produces milkiness when passed through lime water. But this colourless gas does not decolorise acidified $\mathrm{KMn}{0}_4$ • 

(b) [X] dissolved in dilute $\mathrm{HCl}$. On reaction with KI gives a white precipitate (P) and iodine ga .... 

(c) [Y] on roasting at high temperature gives metal (M) and a gas $({\mathrm{G}}_1 )$ which turns starch iodate solution blue. 

(d) [Y] on reaction with dilute $\mathrm{HCl}$ gives a white precipitate (MS) and another gas $\left({\mathrm{G}}_2\right)$ which turns lead acetate solution black and also reacts with gas$({\mathrm{G}}_1 )$ to precipitate colloidal sulphur in presence of moisture. $\mathrm{M}, \mathrm{S}, \left[\mathrm{X}\right], \left[\mathrm{Y}\right]$ gives greenish blue flame.

The gas $({\mathrm{G}}_1 )$ can act as: