# Two particles of mass 2m and 3m are at a distance 'd' apart. Under their mutual gravitational force, they start moving towards each other. The acceleration of their center of mass when they are $d/2$ apart is

$\begin {array} {1 1} (a)\;\frac{12\;Gm}{d^2} & \quad (b)\;\frac{8\;Gm}{d^2} \\ (c)\;\frac{24\;Gm}{d^2} & \quad (d)\;zero \end {array}$