# The polarisability of the halide ions in the salts $CaX_2$ is in the order of
$\begin{array}{1 1}(a)\;F^- < Cl^- < I^- < Br^-\\(b)\;F^- < Cl^- < Br^- < I^-\\(c)\;Cl^- < F^- < Br^- < I^-\\(d)\;I^- < F^- < Cl^- < F^-\end{array}$