A diffraction grating with $N$ lines per metre is used to diffract light of a range of wavelengths $\lambda$. The graph illustrates how the diffraction angle $\theta$ varies with $\lambda$ for several wavelengths in the $n$th order interference pattern. What is the gradient of the graph?
- A$Nn$
- B$\frac{N}{n}$
- C$\frac{n}{N}$
- D$\frac{1}{Nn}$