Two trolleys are kept in contact on a horizontal surface, with a compressed spring placed between them. Once they are released, the trolleys separate from the spring. The trolley with mass $2\,\text{kg}$ travels to the left with a final speed of $2\,\text{m s}^{-1}$. What amount of elastic potential energy was stored in the spring?
- A$4\,\text{J}$
- B$6\,\text{J}$
- C$8\,\text{J}$
- D$12\,\text{J}$