Two equal masses move towards one another along a frictionless track with speeds of $60\,\text{cm s}^{-1}$ and $30\,\text{cm s}^{-1}$. On collision, they remain stuck together. What is the speed of the masses after impact?
- A$15\,\text{cm s}^{-1}$
- B$20\,\text{cm s}^{-1}$
- C$30\,\text{cm s}^{-1}$
- D$45\,\text{cm s}^{-1}$