A colourless solution is tested. (i) Warming the original solution with $\mathrm{NaOH}$ gives a gas that turns Nessler's reagent brown. (ii) After the usual earlier groups, $\mathrm{NH}{}_4\mathrm{Cl}$, $\mathrm{NH}{}_4\mathrm{OH}$ and $(\mathrm{NH}{}_4){}_2\mathrm{CO}{}_3$ give a white precipitate that dissolves completely in hot acetic acid. (iii) The acetic acid solution gives no precipitate with $\mathrm{K}{}_2\mathrm{CrO}{}_4$, but the precipitate imparts a brick-red flame. Which conclusions are justified?

1. A$\mathrm{NH}{}_4{}^{+}$ is present in the original solution.
2. B$\mathrm{Ca}{}^{2+}$ is present.
3. C$\mathrm{Ba}{}^{2+}$ is present.
4. D$\mathrm{Mg}{}^{2+}$ is certainly absent.