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

Fact 2: Polar bears grow fur to survive in the cold.

Given the two facts above, answer the question "How do polar bears stay warm in the winter?" with the following options: 
- They have cold blood
 - protect them
 - They eat a lot
 - hard outer covering
 - by indirect light
 - They go swimming
 - They have thick fur
 - When it's cold


 

       They have thick fur  