A mixture of $\mathrm{Fe}(\mathrm{OH}){}_3$, $\mathrm{Al}(\mathrm{OH}){}_3$ and $\mathrm{Cr}(\mathrm{OH}){}_3$ is boiled with excess $\mathrm{NaOH}$ and $\mathrm{Na}{}_2\mathrm{O}{}_2$ and filtered. Which description is correct?

1. AThe residue is brown $\mathrm{Fe}(\mathrm{OH}){}_3$; the filtrate on boiling with $\mathrm{NH}{}_4\mathrm{Cl}$ gives yellow $\mathrm{PbCrO}{}_4$
2. BThe residue is brown $\mathrm{Fe}(\mathrm{OH}){}_3$; the yellow filtrate holds chromate and aluminate, and boiling it with $\mathrm{NH}{}_4\mathrm{Cl}$ precipitates white $\mathrm{Al}(\mathrm{OH}){}_3$ leaving a yellow solution
3. CThe residue is green $\mathrm{Cr}(\mathrm{OH}){}_3$; the filtrate is colourless aluminate
4. DThe residue is white $\mathrm{Al}(\mathrm{OH}){}_3$; the filtrate is yellow from $\mathrm{FeO}{}_4{}^{2-}$