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

Fact 2: Hobby greenhouses used to be small glass houses or fiberglass buildings.

Given the two facts above, answer the question "Glass or fiberglass buildings are used to protect plants by keeping them what?" with the following options: 
- mild
 - heat
 - hibernating
 - cooled
 - warm
 - warmth
 - cold
 - frozen


 

     warm    