A solid is warmed with ethanol and concentrated $\mathrm{H}{}_2\mathrm{SO}{}_4$ and the vapour is ignited. It burns with a green-edged flame. The anion present is:

1. A$\mathrm{PO}{}_4{}^{3-}$
2. B$\mathrm{SO}{}_4{}^{2-}$
3. C$\mathrm{C}{}_2\mathrm{O}{}_4{}^{2-}$
4. D$\mathrm{BO}{}_3{}^{3-}$ (borate)