[单选题]
A beam of light is incident on a diffraction grating that has $N$ lines per $cm$. An interference pattern is observed on a distant screen. The graph shows the variation of deflection angle $θ$ for the fifth maximum with wavelength $λ$ of the beam.
Which of the following gives the gradient of the graph?
A. $100N\,m^{-1}$
B. $500N\,m^{-1}$
C. $\frac{N}{100}\,m^{-1}$
D. $\frac{N}{500}\,m^{-1}$
