Three identical spheres each of radius 'R' and touching each other so that the centres A,B and C lie on a straight line. The position of their centre of Mass from A is

Question

$$\begin {array} {1 1} (a)\;\frac{2R}{3} \\ (b)\;2R \\ (c)\;\frac{5R}{3} \\ (d)\;\frac{4R}{3} \end {array}$$