An object of mass $20\,\text{kg}$ moves at a steady speed of $6.0\,\text{m s}^{-1}$. It then collides with another object of mass $12\,\text{kg}$ moving at a steady speed of $15\,\text{m s}^{-1}$ in the opposite direction. After the impact, the objects remain stuck together. Determine the speed of the objects immediately after the collision.
- A$1.9\,\text{m s}^{-1}$
- B$9.0\,\text{m s}^{-1}$
- C$9.4\,\text{m s}^{-1}$
- D$21\,\text{m s}^{-1}$