The solubility products of \(\mathrm{NiS}, \mathrm{ZnS}, \mathrm{CdS}\) and \(\mathrm{HgS}\) are \(4.7 \times 10^{-5}, 1.6 \times 10^{-24}, 8 \times 10^{-27}\) and \(4 \times 10^{-53}\) respectively. An aqueous solution contains \(\mathrm{Ni}^{2+}, \mathrm{Zn}^{2+}, \mathrm{Cd}^{2+}\) and \(\mathrm{Hg}^{2+}\) of equal concentration. \(\mathrm{H}_2 \mathrm{~S}\) gas was passed into this solution very slowly. The first and the last ions that precipitate as sulphides are respectively.

1. A \(\mathrm{Ni}^{2+}, \mathrm{Hg}^{2+}\)
2. B \(\mathrm{Hg}^{2+}, \mathrm{Cd}^{2+}\)
3. C \(\mathrm{Zn}^{2+}, \mathrm{Hg}^{2+}\)
4. D \(\mathrm{Hg}^{2+}, \mathrm{Ni}^{2+}\)