Consider an initially neutral hollow conducting spherical shell with inner radius $r$ and outer radius $2r$. $A$ point charge $+Q$ is now placed inside the shell at a distance $r/2$ from the centre. The shell is then grounded by connecting the outer surface to the earth. $P$ is an external point at a distance $2r$ from the point charge $+Q$ on the line passing through the centre and the point charge $+Q$ as shown in the figure. The magnitude of the force on a test charge $+q$ placed at $P$ will be

• A
  $\frac{1}{4 \pi \varepsilon_0} \frac{q Q}{4 r^2}$
• B
  $\frac{1}{4 \pi \varepsilon_0} \frac{9 q Q}{100 r^2}$
• C
  $\frac{1}{4 \pi \varepsilon_0} \frac{4 q Q}{25 r^2}$
• D
  $0$