Two concentric spheres of radii R and r have similar charges with equal surface densities ($\mathrm{\sigma }$). The electric potential at their common centre will be

• A

  $\mathrm{\sigma }/{\mathrm{\epsilon }}_0$

• B

  $\frac{\mathrm{\sigma }}{{\mathrm{\epsilon }}_0}(\mathrm{R}-\mathrm{r})$

• C

  $\frac{\mathrm{\sigma }}{{\mathrm{\epsilon }}_0}(\mathrm{R}+\mathrm{r})$

• D

  none of the above