Two spheres of masses 2M and M are initially at rest at a distance R apart. Due to mutual force of attraction, they approach each other. When they are at separation `R//2`, the acceleration of the centre of mass of spheres would be

Options

A. `0 m//s^(2)`
B. `gm//s^(2)`
C. `3 gm//s^(2)`
D. `12 gm//s^(2)`