Material a is less optically dense than material b. how can you tell from the diagram above?

You can tell because the line bends and the closer it is to horizontal or past horizontal it is more dense

Ithink it is letter b, i may be wrong but i think that is the answer

Velocity=total displacement/timefrom house to store is 500 from store to neighbor is 200 therefore total displacement=500+200=700metrestherefore,700metres/910seconds=0.8metres per second