Light of wavelength l and frequency f passes through a single slit of width a. The diffraction pattern is observed on a screen a distance x from the slit. Which of the following will decrease the width of the central maximum?
选项：

A:Decrease the slit width. ;
B:decrease the frequency f of the light.;
C: increase the wavelength l of the light.;
D:decrease the distance x of the screen from the slit.