Fact 1: Afire requires oxygen to burn.

Fact 2: They produce oxygen as a by-product of photosynthesis.

Given the two facts above, answer the question "A fire requires a by-product of what?" with the following options: 
- sunlight
 - heat energy
 - electromagnetic energy
 - photosynthesis.
 - evaporation
 - heating liquids
 - Electrical energy
 - harmful substances


 

    photosynthesis.     