From the following sparingly soluble salts, identify the salt with maximum solubility 

$\begin{array}{l}\text{'}\mathrm{AB}\text{'}; \mathrm{S}={10}^{-8}\mathrm{M}\\ \text{'}\mathrm{PQ}\text{'}; \mathrm{S}={10}^{-7}\mathrm{M}\\ \text{'}\mathrm{XY}\text{'}; \mathrm{S}=2\times {10}^{-7}\mathrm{M}\\ \text{'}\mathrm{YZ}\text{'}; \mathrm{S}=3\times {10}^{-8};\\ \mathrm{For} \mathrm{all} \mathrm{these} \mathrm{salts} {\mathrm{K}}_{\mathrm{sp}}={\mathrm{S}}^2;\\ \sqrt{{\mathrm{K}}_{\mathrm{sp}}}=\mathrm{S};\end{array}$