Elastic and plastic behaviour — practice question

The graph plots how the length of a spring changes as the load applied to it increases. What is the spring constant of the spring?

• A$0.080\,\text{N m}^{-1}$
• B$0.13\,\text{N m}^{-1}$
• C$2.7\,\text{N m}^{-1}$
• D$8.0\,\text{N m}^{-1}$