A spring that follows Hooke’s law has an unstretched length of $10\,\text{cm}$. A $20\,\text{N}$ load is hung from the spring. Its length then becomes $36\,\text{cm}$. What is the spring constant $k$ for the spring?
- A$0.56\,\text{N cm}^{-1}$
- B$0.77\,\text{N cm}^{-1}$
- C$1.3\,\text{N cm}^{-1}$
- D$1.8\,\text{N cm}^{-1}$