A point mass m is placed inside a spherical shell of radius R and mass M at a distance `R/2` form the centre of the shell. The gravitational force exerted by the shell on the point mass is
A. `(2Gm^(2))/(R^(2))`
B. `(Gm^(2))/(R^(2))`
C. `(Gm^(2))/(2R)`
D. zero