$A$ 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
  $\frac{Q}{2\pi \varepsilon_0 R}$
• B
  $\frac{3Q}{2\pi \varepsilon_0 R}$
• C
  $\frac{3Q}{4\pi \varepsilon_0 R^2}$
• D
  $\frac{4Q}{4\pi \varepsilon_0 R^2}$