- Photosynthesis means green plants convert from carbon dioxide and water and solar energy into carbohydrates and food and oxygen for themselves
- If a process converts something to something else, then that process is a source of that something else
- Therefore, photosynthesis is a source of oxygen

- Photosynthesis is a source of oxygen
- Plants perform photosynthesis
- So, plants are a source of oxygen through photosynthesis

- Plants are a source of oxygen through photosynthesis
- If something releases something else then that something is the source of that something else
- So, plants release oxygen through photosynthesis

- Plants release oxygen through photosynthesis
- A tree is a kind of plant
- So, trees release oxygen through photosynthesis

So, trees release oxygen through photosynthesis
Q: Cindy learned that trees release gases into to the air during photosynthesis. What gas does a tree release?
Context:
- Photosynthesis means green plants convert from carbon dioxide and water and solar energy into carbohydrates and food and oxygen for themselves 
- If something releases something else then that something is the source of that something else 
- Plants perform photosynthesis 
- If a process converts something to something else, then that process is a source of that something else 
- A tree is a kind of plant