A hollow spherical shell of radius r has a uniform charge density $\raisebox{1ex}{$\sigma $}\!\left/ \!\raisebox{-1ex}{$\pi $}\right.$. It is kept in a cube of edge 3r such that the centre of the cube coincides with the centre of the shell. The electric flux that comes out of a face of the cube is 

• A

  $\frac{2r^2\sigma }{3{\epsilon }_0}$

• B

  $\frac{r^2\sigma }{3{\epsilon }_0}$

• C

  $\frac{4r^2\sigma }{3{\epsilon }_0}$

• D

  $\frac{r^2\sigma }{6{\epsilon }_0}$