Two solid spheres make up an isolated system. Sphere X is travelling at a speed of $6\,\text{cm s}^{-1}$ along a straight path directly towards sphere Y, which is stationary, as illustrated. The spheres undergo a perfectly elastic collision. Once the collision is over, sphere X continues in the same direction as before the collision but now with a speed of $2\,\text{cm s}^{-1}$. What is the speed of sphere Y?
- A$2\,\text{cm s}^{-1}$
- B$4\,\text{cm s}^{-1}$
- C$6\,\text{cm s}^{-1}$
- D$8\,\text{cm s}^{-1}$