light-year, astronomical unit, kilometer, angstrom angstrom, kilometer, astronomical unit, light-year astronomical unit, angstrom, kilometer, light-year kilometer, angstrom, light-year, astronomical unit
I gave my students this multiple choice question: Which of the following lists units of length from smallest to largest?

Only one answer is correct among these 4 choices:
- light-year, astronomical unit, kilometer, angstrom
- angstrom, kilometer, astronomical unit, light-year
- astronomical unit, angstrom, kilometer, light-year
- kilometer, angstrom, light-year, astronomical unit

Could you tell me which one is correct?

angstrom, kilometer, astronomical unit, light-year