[单选题]
Light with wavelength λ is incident on a diffraction grating. The maximum number of bright fringes that can be observed on the screen is 9. Which of the following gives the number of slits per cm in the grating?
A. $\frac{1}{400λ}\,cm^{-1}$
B. $\frac{1}{4λ}\,cm^{-1}$
C. $4λ\,cm^{-1}$
D. $400λ\,cm^{-1}$
