Fact 1: Agreenhouse is used to protect plants by keeping them warm.

Fact 2: Greenhouse gases act as the glass in a greenhouse.

Given the two facts above, answer the question "what keeps plants warm?" with the following options: 
- glass
 - trees
 - lipids
 - fats
 - fibers
 - keratin
 - energy
 - heat


 

 glass        