Consider the case of single-slit Fraunhofer diffraction, where the diffraction angle of the central maximum is small. If the width of slit increases to 150%, and the wavelength of the illuminated monochromatic light decrease to 75%, then the width of the central maximum increases/decrease to ().
选项：

A:3/4.;
B:2/3. ;
C: 9/8. ;
D:1/2.