There are three concentric thin spheres of radius a, b, c (a > b > c). The total surface charge densities on their surfaces are $\mathrm{\sigma },-\mathrm{\sigma },\mathrm{\sigma }$ respectively. The magnitude of electric field at r ( distance from centre) such that a > r > b is

• A

  0

• B

  $\frac{\mathrm{\sigma }}{{\mathrm{\epsilon }}_0{\mathrm{r}}^2}\left({\mathrm{b}}^2-{\mathrm{c}}^2\right)$

• C

  $\frac{\mathrm{\sigma }}{{\mathrm{\epsilon }}_0{\mathrm{r}}^2}\left({\mathrm{a}}^2+{\mathrm{b}}^2\right)$

• D

  none of these