- Predators threaten the existance of other animals
- Threatening the existence of an animal has a negative impact on that animal
- Therefore, predators have a negative impact on an animal's survival

- Predators have a negative impact on an animal's survival
- Avoiding something with a negative impact has a positive impact
- Therefore, avoiding predators has a positive impact on an organism 's survival

- Avoiding predators has a positive impact on an organism 's survival
- Defenses can help animals avoid predators
- Therefore, organisms will use defenses as a reseponse to predators

- Quills are a kind of defense mechanism
- Quills are part of a porcupine
- So, quills are part of a porcupine used for defense

- Organisms will use defenses as a reseponse to predators
- Quills are part of a porcupine used for defense
- Therefore, a porcupine will use quills as a response to predators

Therefore, a porcupine will use quills as a response to predators
Question: Which stimulus causes a porcupine to extend its quills?
Context: - Predators threaten the existance of other animals 
- Defenses can help animals avoid predators 
- Avoiding something with a negative impact has a positive impact 
- Threatening the existence of an animal has a negative impact on that animal 
- Quills are a kind of defense mechanism 
- Quills are part of a porcupine