A diffraction grating with $N$ lines per metre is used to deflect light of different wavelengths $\lambda$. The graph plots the relationship between the deflection angle $\theta$ and $\lambda$ for a range of wavelengths in the $n^{\text{th}}$ order interference pattern. What is the gradient of the graph?
- A$Nn$
- B$\frac{N}{n}$
- C$\frac{n}{N}$
- D$\frac{1}{Nn}$