A moving body of mass $m$ and velocity $3 \mathrm{~km} / \mathrm{h}$ collides with a rest body of mass 2 m and stick to it. Now the combined mass starts to move. What will be the combined velocity?

• A

  $4 \mathrm{~km} / \mathrm{h}$
• Correct answer
  B

  $1 \mathrm{~km} / \mathrm{h}$
• C

  $2 \mathrm{~km} / \mathrm{h}$
• D

  $3 \mathrm{~km} / \mathrm{h}$

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