A tunnel is dug along a diameter of the earth. If M_e and R. are the mass and radius, respectively, of the earth, then the force on a particle of mass m placed in the tunnel at a distance r from the centre is 

• A

  $\frac{{\mathrm{GM}}_{\mathrm{e}}\mathrm{m}}{{\mathrm{R}}_{\mathrm{e}}^3}\mathrm{r}$

• B

  $\frac{{\mathrm{GM}}_{\mathrm{e}}\mathrm{m}}{{\mathrm{R}}_{\mathrm{e}}^3\mathrm{r}}$

• C

  $\frac{{\mathrm{GM}}_{\mathrm{e}}{\mathrm{mR}}_{\mathrm{e}}^3}{\mathrm{r}}$

• D

  $\frac{{\mathrm{GM}}_{\mathrm{e}}\mathrm{m}}{{\mathrm{R}}_{\mathrm{e}}^2}\mathrm{r}$