[单选题]
Ray diagrams can be use78czs/ukfd-(l4n5hn :m f j0qzp;u s hd to s,z3u 9lxlz nxl9b xk/dq81z ;uhow the reflection, refraction and transmission of light atkxbnzzlz1/ 9l qd3l;xu8ux,9 the boundary between media.
In the diagram, what feature does $X$ represent?
A. Reflected ray
B. Refractedray
C. Transmitted ray
D. Incident ray