When the sample of copper with zinc impurity is to be purified by electrolysis, the appropriate electrodes are :

For a cell given below


$$ \begin{aligned} \mathrm{Ag}^{+}+\mathrm{e}^{-} & \longrightarrow \mathrm{Ag}; E^{\circ}=x \\\\ \mathrm{Cu}^{2+}+2 e^{-} & \longrightarrow \mathrm{Cu}{;} E^{\circ}=y \end{aligned} $$

$$ E^{\circ} \text { cell is } $$ :