Hi, please explain this instrumental analysis problem for me! The red lines are the answer for this problem. But I don't understand why they got 50 nanometers for B and 5 micrometers for C. And for A, they got 500 nm. Please explain for me how they got those numbers!

a. (5 pts) You are charged with choosing a grating for the spectrometer above which will be operated from 400-800 nm. You can choose between three gratings with different groove density a) 2000 grooves /mm b) 20,0000 groves /mm and c) 200 grooves /mm. Based on the fundamental properties of diffraction, which one should you choose? Why? A is the best choice based on fundamental properties of diffraction. Diffraction is maximal as the distance between slits/lines approaches the wavelength of light. For B, the grooves are ca. 50nm apart and for C they are ca. 5m, whereas for A they are ca. 500nm - near the wavelength range of interest