Which of the following single slit diffraction experiments has the smallest size of the angle from the center of the diffraction pattern to the first dark fringe? （ ） A. wavelength 400 nm, slit width 0.20 mm; B. wavelength 600 nm, slit width 0.20 mm; C. wavelength 400 nm, slit width 0.30 mm; D. wavelength 600 nm, slit width 0.40 mm.