What do astronomers mean by light pollution?
a. light pollution refers to light used for human activities that brightens the sky and hinders astronomical observations.
b. light pollution refers to harmful gases emitted by common street lights.
c. light pollution refers to pollution caused by light industry as opposed to heavy industry.
d. light pollution is another name for sunlight, which makes it impossible to see stars in the daytime.
e. light pollution refers to the lights that must be used inside major observatories and that make it difficult for astronomers' eyes to adapt to darkness.

a. Light pollution refers to light used for human activities that brightens the sky and hinders astronomical observations.

Explanation:

Light pollution is due to the excessive and misdirected use of artificial light. Light bulbs are often design in an incorrect way, since a great part of its light is not completely directed to the ground and an important percentage is emitted to the sky in where will be scattered and reflected back to ground by the particles in the atmosphere. That brings as an effect a sky glow, therefore the visibility of astronomical objects will be extremely reduce.  

Hence, professional astronomical research and amateur observations will be affected. Light pollution has a negative impact on bird migration at night and in the health of difference species, humans also.          

Explanation:

Letting you Know That  The Answer Was Wrong LYK AKA Letting You Know :)

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.