—Do you know _______? 

—PSP. [ ]

A. what is it called

B. how it is called

C. how it calls

D. what it is called

(2011年上海宝山区模拟)在光的单缝衍射实验中可观察到清晰的亮暗相间的图样，下列四幅图片中属于光的单缝衍射图样的是(　　)

A．a、c　　　　　　　　

B．b、c

C．a、d

D．b、d