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

Fact 2: Angiosperms are the flowering plants.

Given the two facts above, answer the question "A greenhouse is used to protect angiosperms how?" with the following options: 
- their hosts
 - by keeping them underground
 - by keeping them warm
 - by keeping them outside
 - by keeping them exposed
 - Exfoliation
 - organisms and their habitat
 - In the winter


 

   by keeping them warm      