Fact 1: Some animals' fur grows thicker for staying warm in the winter.

Fact 2: Mammals have hair and fur.

Given the two facts above, answer the question "Why do some animals have hair that grows longer during the winter?" with the following options: 
- It expands
 - to make braids
 - to wear a scarf
 - to attract mates
 - When it's cold
 - to stay warm
 - It remains dormant
 - genetic diversity


 

      to stay warm   