-Can you tell me _____________?

-Yesterday afternoon. [ ]

A. when they started

B. when did they start

C. where they lived

D. where did they live

如图甲是α、β、γ三种射线穿透能力的示意图，图乙是工业上利用射线的穿透性来检查金属内部的伤痕的示意图，则图乙中的检查利用的射线是______，请说明利用射线来检查金属内部伤痕的原理，并提出注意事项______．