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

- Plants produce oxygen through photosynthesis
- If something is given off of something , then something is the product of something
- Hence, plants give off oxygen through photosynthesis

Hence, plants give off oxygen through photosynthesis
Question: Which gas is given off by plants?
Context: - Plants perform photosynthesis 
- If a process converts something into something else, then that process is a source of that something else 
- If something is given off of something , then something is the product of something 
- Photosynthesis means green plants convert from carbon dioxide and water and solar energy into carbohydrates and food and oxygen for themselves 
- A source of something produces that something