- Oxygen can be found in air
- An animal requires oxygen for to breathe
- Therefore, an animal requires oxygen in air to breathe

- An animal requires oxygen in air to breathe
- Air quality is a measure of how breathable air is
- Hence, oxygen has a positive impact on air quality

- A source of something produces that something
- Plants are a source of oxygen through photosynthesis
- So, plants produce oxygen through photosynthesis

- Oxygen has a positive impact on air quality
- Plants produce oxygen through photosynthesis
- So, plants have a positive impact on air quality by producing oxygen through photosynthesis

So, plants have a positive impact on air quality by producing oxygen through photosynthesis
Q: How can plants most likely improve air quality?
Context:
- A source of something produces that something 
- Plants are a source of oxygen through photosynthesis 
- Oxygen can be found in air 
- Air quality is a measure of how breathable air is 
- An animal requires oxygen for to breathe