The spring constant $k$ of a coiled wire spring is described by the formula $k = \dfrac{G r^4}{4 n R^3}$, where $r$ represents the radius of the wire, $n$ is the number of wire turns and $R$ is the radius of each turn of the wire. The quantity $G$ depends on the material that the wire is made from. What is a suitable unit for $G$?
- A$\text{N m}^{-2}$
- B$\text{N m}^{-1}$
- C$\text{N m}$
- D$\text{N m}^2$