Assertion (A): In group III, solid or aqueous $\mathrm{NH}{}_4\mathrm{Cl}$ is added before $\mathrm{NH}{}_4\mathrm{OH}$, and $\mathrm{Mg}{}^{2+}$ then stays in solution. Reason (R): $\mathrm{NH}{}_4\mathrm{Cl}$ suppresses ionisation of $\mathrm{NH}{}_4\mathrm{OH}$ by the common-ion effect, so $[\mathrm{OH}{}^{-}]$ is low enough that $\mathrm{Mg}(\mathrm{OH}){}_2$ (larger $K_{sp}$) is not precipitated while $\mathrm{Fe}(\mathrm{OH}){}_3$, $\mathrm{Al}(\mathrm{OH}){}_3$ and $\mathrm{Cr}(\mathrm{OH}){}_3$ (very small $K_{sp}$) are.

1. ABoth A and R are true and R is the correct explanation of A
2. BBoth A and R are true but R is not the correct explanation of A
3. CA is true but R is false
4. DA is false but R is true