Three identical spheres each of mass ‘m’ and radius ‘R’ are placed touching each other so that their centres A, B and C lie on a straight line. The position of their centre of mass from centre of A is

• A

  $\frac{2\mathrm{R}}{3}$

• B

  2R

• C

  $\frac{5\mathrm{R}}{3}$

• D

  $\frac{4\mathrm{R}}{3}$