Here's a problem to solve: One astronomical unit is the average distance between the Earth and the Sun. This unit is most often used to describe the distance between which two objects?

Among the 4 following options, which is the correct answer?

- A: from galaxy to galaxy
 
- B: from Saturn to Mercury
 
- C: from the Sun to Proxima Centauri
 
- D: from the Big Dipper to the Little Dipper
 