Two solid spheres make up an isolated system. Sphere X is travelling in a straight line at a speed of $6\,\text{cm s}^{-1}$ directly towards a sphere Y that is at rest, as illustrated. The collision between the spheres is perfectly elastic. Once the collision is over, sphere X continues in the same direction as before, but now at 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$8\,\text{cm s}^{-1}$