Earth and the Sun. the Moon and the Sun. Earth and the Moon. the Sun and the nearest star.
I gave my students this multiple choice question: An astronomical unit (AU) refers to the distance between

Only one answer is correct among these 4 choices:
- Earth and the Sun.
- the Moon and the Sun.
- Earth and the Moon.
- the Sun and the nearest star.

Could you tell me which one is correct?

Earth and the Sun.