Fact 1: Insulated material can be used for keeping warm.

Fact 2: Sunlight heats the ground, warming it up.

Given the two facts above, answer the question "How can heat be kept?" with the following options: 
- an electric stove
 - exposed material
 - insulated material
 - hydration
 - permeable material
 - electricity
 - wet material
 - In the winter


 

   insulated material      