<p>When&nbsp;<span class="equation">\(FeCr_2O_4\)</span>&nbsp;is fused with <span class="equation">\(Na_2CO_3\)</span>&nbsp;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 KCI forms an orange coloured compound (C). An acidified solution of compound (C) oxidises&nbsp;<span class="equation">\(Na_2SO_3\)</span>&nbsp; to (D). Identify (A), (B), (C) and (D).​​​​​​​</p>