A variable force stretches a spring to several different lengths. The graph shows how the force magnitude changes with the length by which the spring is stretched. What is the spring constant $k$ of the spring?
- A$0.27\,\text{N cm}^{-1}$
- B$0.40\,\text{N cm}^{-1}$
- C$2.5\,\text{N cm}^{-1}$
- D$3.8\,\text{N cm}^{-1}$