A solid metallic sphere has a charge +3Q. Concentric with this sphere is a conducting spherical shell having charge -Q. The radius of the sphere is a and that of the spherical shell is b(b>a). What is the electric field at a distance R(a<R<b) from the centre.

• A

  $\frac{Q}{2{\mathrm{\pi \epsilon }}_0\mathrm{R}}$

• B

  $\frac{3Q}{2{\mathrm{\pi \epsilon }}_0\mathrm{R}}$

• C

  $\frac{3Q}{4{\mathrm{\pi \epsilon }}_0{\mathrm{R}}^2}$

• D

  $\frac{4Q}{4{\mathrm{\pi \epsilon }}_0{\mathrm{R}}^2}$