The electric field at a distance $r$ from the centre in the space between two concentric metallic spherical shells of radii $r_1$ and $r_2$ carrying charges $Q_1$ and $Q_2$ respectively is $(r_1 < r < r_2)$.

• A
  $\frac{Q_1 + Q_2}{4\pi \epsilon_0 (r_1 + r_2)^2}$
• B
  $\frac{Q_1 + Q_2}{4\pi \epsilon_0 r^2}$
• C
  $\frac{Q_1}{4\pi \epsilon_0 r^2}$
• D
  $\frac{Q_2}{4\pi \epsilon_0 r^2}$