When $\mathrm{FeCr}_2\mathrm{O}_4$ is fused with $\mathrm{Na}_2\mathrm{CO}_3$ in the presence of air it gives a yellow solution of compound (A). Compound (A) on acidification gives compound (B). Compound (B) on reaction with $\mathrm{KCl}$ forms an orange coloured compound (C). An acidified solution of compound (C) oxidises $\mathrm{Na}_2\mathrm{SO}_3$ to (D). Identify (A), (B), (C) and (D).