Rank each set of ions in order of decreasing size: Ca$^{2+}$, Sr$^{2+}$, Mg$^{2+}$.

Question

$\begin{array}{1 1} Sr^{2+} > Ca^{2+} > Mg^{2+}. \\ Ca^{2+} > Sr^{2+} > Mg^{2+}. \\ Mg^{2+} > Sr^{2+} > Ca^{2+}.\\ Ca^{2+} > Mg^{2+} > Sa^{2+}. \end{array} $