What Do We Mean by the Frost Line When We

What do we mean by the frost line when we discuss the formation of planets in the solar nebula?

A) It marks the special distance from the Sun at which hydrogen compounds become abundant; closer to the Sun, there are no hydrogen compounds.
B) It is a circle at a particular distance from the Sun, beyond which the temperature was low enough for ices to condense.
C) It is another way of stating the temperature at which water freezes into ice.
D) It is the altitude in a planet's atmosphere at which snow can form.