Three objects $A, B$ and $C$ are kept in a straight line on a frictionless horizontal surface. The masses of $A, B$ and $C$ are $m, 2m$ and $2m$ respectively. $A$ moves towards $B$ with a speed of $9 \mathrm{m} {\mathrm{s}}^{-1}$ and makes an elastic collision with it. Thereafter $B$ makes a completely inelastic collision with $C.$ All motions occur along the same straight line. The final speed of $C$ is :

• A

  $6 \mathrm{m} {\mathrm{s}}^{-1}$
• B

  $9 \mathrm{m} {\mathrm{s}}^{-1}$
• C

  $4 \mathrm{m} {\mathrm{s}}^{-1}$
• Correct answer
  D

  $3 \mathrm{m} {\mathrm{s}}^{-1}$

Hence, the correct answer is option: 'D'.