A mass m is placed in the cavity inside a hollow sphere of mass M as shown in fig. What is the gravitational 

force on the mass m ?

• A

  $\frac{\mathrm{GMm}}{{\mathrm{R}}^2}$

• B

  $\frac{\mathrm{GMm}}{{\mathrm{r}}^2}$

• C

  $\frac{\mathrm{GMm}}{\left({\mathrm{R}}^2-{\mathrm{r}}^2\right)}$

• D

  zero