Fact 1: Threatening behavior by predators causes a porcupine to extend its quills.

Fact 2: Aggression is active, harmful and offensive, threatens all and benefits none.

Given the two facts above, answer the question "What by predators cause porcupines to extend their quills?" with the following options: 
- hypothyroidism
 - killing insects
 - kisses
 - aggression
 - love
 - hyperthyroidism
 - kinetic energy
 - cohabitating


 

    aggression     