What does the speaker mean a. He has to answer a lot of phone calls in the office, b. He has to help his colleagues to ask for leave. c. He is busy with his work. d. He is getting sick.

在杨氏双缝干涉实验中，如果用薄云母片盖住下缝，则干涉条纹将（）。

A.条纹上移，干涉条纹之间的距离不变

B.条纹上移，干涉条纹之间的距离变小

C.条纹下移，干涉条纹之间的距离不变

D.条纹下移，干涉条纹之间的距离变大