A spring with an initial length of $100\ \text{mm}$ is being shortened by a force. The graph illustrates how the compressive force $F$ changes with the spring length $L$. How much energy is stored in the spring when the length is $70\ \text{mm}$?
- A$0.090\ \text{J}$
- B$0.21\ \text{J}$
- C$0.27\ \text{J}$
- D$0.63\ \text{J}$