D) Solid Waste
(I'm not 100% sure, let me know if I made a mistake :))
Answer:
It is the angle of refraction in glass for which the angle of incidence in air is 90°.
Explanation:
When light is refracted, the sine of the angle changes in proportion to the refractive index. When the light is 90 degrees, the sine is zero, and zero is still zero on the other side. So nothing changes.
When a valence electron shell becomes full, it will usually become stable.