Its c on plato. My teacher told me.
because he said that two conditions must be met:
the light is in the more dense medium and approaching the less dense medium.
the angle of incidence is greater than the so-called critical angle(no more than 90 degrees).
he also told me that the more dense something is the faster its atoms move, and the less dense something is the slower it moves. So if the light is moving from a point more dense, to less dense, it must be c.