In the Solar System, What Is the Snow Line

In the solar system, what is the snow line?

A) either one of a pair of lines on any planet, parallel to its equator, between which snow is never found
B) a boundary in the solar system such that ices of water, carbon dioxide, methane, and ammonia are found only outside of this line (i.e., farther from the Sun)
C) a boundary in the solar system such that all fluids outside of this line (i.e., farther from the Sun) must be frozen
D) a boundary within the asteroid belt which divides the asteroids into those composed of rock and those composed of ice