- A galaxy contains dusts / stars / planets
- If something contains something else, then that something is made of that something else
- Therefore, a galaxy is made of dusts, stars, and planets

- A galaxy is made of dusts, stars, and planets
- A galaxy is held together by gravity
- Therefore, a galaxy is made of dusts, stars, and planets held together by gravity

So, a galaxy is made of dusts, stars, and planets held together by gravity
Q: Which statement is the best description of the Milky Way galaxy?
Context:
- A galaxy contains dusts / stars / planets 
- If something contains something else, then that something is made of that something else 
- A galaxy is held together by gravity