$A$ solid metal sphere has a charge of $+3Q$. It is concentric with a conducting spherical shell having a charge of $-Q$. The radius of the sphere is $a$ and the radius of the shell is $b$ $(b > a)$. The electric field at a distance $R$ from the center $(a < R < b)$ is .......

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