The mass density of a solid sphere is $\rho$. The radius of the sphere is $R$. The gravitational field at a distance $r$ from the centre of the sphere inside it is:

• A
  $\frac{4\pi G\rho r}{3}$
• B
  $\frac{4\pi G\rho r^2}{3}$
• C
  $\frac{4\pi G\rho R^3}{3r^2}$
• D
  $\frac{\rho G R^3}{\pi r}$