$A$ positive charge $Q$ is placed on a conducting spherical shell with inner radius $R_1$ and outer radius $R_2$. $A$ particle with charge $q$ is placed at the center of the spherical cavity. The magnitude of the electric field at a point in the cavity,at a distance $r$ from the center,is

• A
  zero
• B
  $\frac{Q}{4 \pi \varepsilon_0 r^2}$
• C
  $\frac{q}{4 \pi \varepsilon_0 r^2}$
• D
  $\frac{(Q+q)}{4 \pi \varepsilon_0 r^2}$