What Is Important About the Habitable Zone Around a Star

What is important about the habitable zone around a star that makes astronomers believe life could exist there?

A) The level of UV radiation from the star is minimal in the habitable zone.
B) Only in the habitable zone can complex organic molecules be found.
C) Planets in the habitable zone will always have an oxygen atmosphere.
D) In the habitable zone, liquid water can exist on the surface for long periods of time