What do astronomers mean by light pollution?
A) Light pollution is a type of air pollution created by lightweight gases such as hydrogen and helium.
B) Light pollution is a term used to describe the appearance of the sky in regions that are crowded with stars.
C) Light pollution means contamination of light caused by chemicals in the Earth's atmosphere.
D) Light pollution is light from human sources that makes it difficult to see the stars at night.
Answer: D