According to Moseley's law, the frequency (v) of the $K_{\alpha}$ line and the atomic number Z of the element have the relation (A and B are constants);

Question

$$\begin {array} {1 1} (a)\;\frac{v}{(Z-A)}=B & \quad (b)\; \frac{\sqrt v}{(Z-A)}=B \\ (c)\;v(Z-A)=B & \quad (d)\;v(Z-A)^2=B \end {array}$$