To a solution of a colourless efflorescent sodium salt, when dilute acid is added, a colourless gas is evolved along with formation of a white precipitate. Acidified dichromate solution turns green when the colourless gas is passed through it. The sodium salt is
(A) \(\mathrm{Na}_{2} \mathrm{SO}_{3}\)
(B) \(\mathrm{Na}_{2} \mathrm{~S}\)
(C) \(\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\)
(D) \(\mathrm{Na}_{2} \mathrm{~S}_{4} \mathrm{O}_{6}\)