What distance was used to define the astronomical unit (AU)?
A) the average distance across the sun, its diameter
B) the average distance from the sun to Mercury, the closest planet
C) the average distance from the sun to Earth, our home planet
D) the average distance from the sun to Neptune, the farthest planet

The average distance from the sun to Earth

Explanation:

Astronomical unit is defined as the the average distance from the center of the earth to the center of the sun.It is represented by AU.The value is equal to a unit of measurement of  149.6 million kilometers. An Astronomical Unit  is approximately equal to  93 million miles (150 million km). It's approximately 8 light-minutes.Astronomical units are usually used to measure distances in solar system.

It allows water to resist freezing and thawing cycle.