The ions $\mathrm{S}{}^{2-}$, $\mathrm{Cl}{}^{-}$, $\mathrm{K}{}^{+}$ and $\mathrm{Ca}{}^{2+}$ are isoelectronic with Slater $Z^{\ast }$ values (3p electron) of 4.75, 5.75, 7.75 and 8.75 respectively. With radius assumed proportional to $1/Z^{\ast }$, which statements are correct?

1. AThe difference $Z^{\ast }(\mathrm{K}{}^{+})-Z^{\ast }(\mathrm{Cl}{}^{-})$ equals the difference in nuclear charge, $2.00$.
2. B$\mathrm{Ca}{}^{2+}$ is the smallest and $\mathrm{S}{}^{2-}$ the largest of the four.
3. C$\mathrm{K}{}^{+}$ is larger than $\mathrm{Cl}{}^{-}$ because it has more protons.
4. DThe predicted ratio $r(\mathrm{S}{}^{2-})/r(\mathrm{Ca}{}^{2+})$ is 0.54.