Consider an atom with atomic number $Z$ as consisting of a positive point charge at the centre and surrounded by a distribution of negative electricity uniformly distributed within a sphere of radius $R$. The electric field at a point inside the atom at a distance $r$ from the centre is

• A
  $\frac{Ze}{4\pi\varepsilon_0}\left[\frac{1}{r^2}-\frac{r}{R^3}\right]$
• B
  $\frac{Ze}{4\pi\varepsilon_0}\left[\frac{1}{r^2}+\frac{1}{R^3}\right]$
• C
  $\frac{2Ze}{4\pi\varepsilon_0 r^2}$
• D
  $0$