A body of mass $m$ travelling with velocity $9.0\,\text{m s}^{-1}$ undergoes a head-on elastic collision with another body of mass $2m$ that is initially at rest. Following the collision, both bodies are moving. No external forces act on the system. What is the velocity of the body of mass $2m$ after the collision?
- A$-6.0\,\text{m s}^{-1}$
- B$-3.0\,\text{m s}^{-1}$
- C$4.5\,\text{m s}^{-1}$
- D$6.0\,\text{m s}^{-1}$