If the distance from earth to alpha centauri is 4.34 light-years, then a) alpha centauri is the closest star to earth. b) we see alpha centauri as it was four years ago. c) the distance is equal to 4.34 astronomical units. d) alpha centauri is four times farther from earth than the sun.

We see alpha centauri as it was 4.34 years ago.

glen is correct.

step-by-step explanation:

glen is correct because when you multiply 4 * 2 = 8.

and 4 * 3 = 12.

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.