Two particles having masses m and 2m are travelling along x-axis on a smooth surface with velocities u₁ and u₂, collide. If their velocities after collision are v₁ and v₂, then the ratio of velocities of their centre of mass before and after impact is

• A

  2 : 1

• B

  2 : 3

• C

  1 : 1

• D

  1 : 2