Fact 1: Energy is used for heating a building by a heating system.

Fact 2: Passive solar thermal energy involves heating buildings using direct sunlight.

Given the two facts above, answer the question "A method of heating a building might be what?" with the following options: 
- dangerous
 - Blizzards
 - Heated arguments
 - threatened
 - major threat to health
 - Direct sunlight.
 - Mothballs
 - slow and inefficient


 

      Direct sunlight.   