Explain why Earth does not have as many craters as Mars, Mercury, and the Moon.

The reason is mainly because Earth  has an atmosphere.

Explanation:

We don't see as many craters in our planet as most of them have been eroded away by wind, water, volcanic activity or any other forces. Earth is also surrounded by an atmosphere which other planets like Mars, Mercury and the Moon lacks. The atmosphere reduces the impact of meteoroids when they hit the surface of a planet as the atmosphere disintegrates them into smaller particles. Also, the atmosphere creates a planet's own weather condition, no atmosphere means no weather. So, planets like Mars, Mercury where there is no atmosphere, there is no weather to erode away the craters. Hence, most of them are still visible on their surfaces.

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.

it's a b and