. The index of refraction is the ratio of the speed of light .

in a material to the density of the material

in the material to the speed of light in a vacuum

in a vacuum to the speed of light in the material

in a vacuum to the speed of light in the material

Explanation:

The index of refraction, also known as the refractive index, is the ratio of the speed of light in a vacuum to the speed of light in the material.

When light rays travel travels from one medium to another, it bends as a result of a change in the speed of traveling from the first medium to the second medium.

The refractive index (n) of a material is calculated by:

                     sin i/sin r

Where i = the incidence angle of the traveling ray in the vacuum and r = angle of refraction.

The refractive index (n) is also given as:

                         c/v

Where c = velocity of light in the vacuum and v = velocity of light in the material.

i am positive it is c. condensing but i may be wrong

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."