Some animals' fur grows thicker for staying warm in the winter, and polar bears grow fur to survive in the cold. Given these facts, how do polar bears stay warm in the winter among 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  