A small conducting sphere of the radius $r$ is lying concentrically inside a bigger hollow conducting sphere of radius $R$. The bigger and smaller sphere are charged with $\mathrm{Q}$ and $q (\mathrm{Q}\mathrm{ }> q)$ and are insulated from each other. The potential difference between the sphere will be

• Correct answer
  A

  $\frac{1}{4\mathrm{\pi }{\mathrm{\epsilon }}_0}\mathrm{ }\left(\frac{\mathit{q}}{\mathit{r}}–\frac{\mathit{q}}{\mathit{R}}\right)$
• B

  $\frac{1}{4\mathrm{\pi }{\mathrm{\epsilon }}_0}\mathrm{ }\left(\frac{\mathit{q}}{\mathit{R}}–\frac{\mathit{Q}}{\mathit{r}}\right)$
• C

  $\frac{1}{4\mathrm{\pi }{\mathrm{\epsilon }}_0}\mathrm{ }\left(\frac{\mathit{q}}{\mathit{r}}–\frac{\mathit{Q}}{\mathit{R}}\right)$
• D

  $\frac{1}{4\mathrm{\pi }{\mathrm{\epsilon }}_0}\mathrm{ }\left(\frac{\mathit{Q}}{\mathit{R}}+\frac{\mathit{q}}{\mathit{r}}\right)$

Hence, the correct answer is option: 'A'.