To a freshly prepared solution of $\mathrm{FeSO}{}_4$, dilute acetic acid is added and then the test solution; a brown colour appears immediately, without any concentrated sulphuric acid. The test solution contains

1. A$\mathrm{Br}{}^{-}$
2. B$\mathrm{NO}{}_3{}^{-}$
3. C$\mathrm{NO}{}_2{}^{-}$
4. D$\mathrm{SO}{}_3{}^{2-}$