Fact 1: Energy enters ecosystems in the form of sunlight or chemical compounds.

Fact 2: Sunlight is energy released from fusion reactions in the sun.

Given the two facts above, answer the question "What do ecosystems use as energy?" with the following options: 
- Nutrients
 - chemicals
 - heat
 - wind
 - trees
 - food
 - glucose
 - fusion


 

        fusion 