Why do astronomers use astronomical units (AU) instead of kilometers (km)?

Question

A:AU is used for spacing, while km is used for length.
B:AU is used in astronomy, while km is used in physics
C:Large distances are easier to express in Au than in KM.
D:There are no km in outer space, only AU.

Answer 1

C: Large distances are easier to express in Au than in KM.