Two masses of equal mass move towards one another on a frictionless air track at speeds of $60\,\text{cm s}^{-1}$ and $40\,\text{cm s}^{-1}$. They become joined when they collide. What is the speed of the masses immediately after the collision?
- A$10\,\text{cm s}^{-1}$
- B$20\,\text{cm s}^{-1}$
- C$40\,\text{cm s}^{-1}$
- D$50\,\text{cm s}^{-1}$