The food chain ? 
Sun
Primary producers
Primary consumers
Secondary consumers 
Tertiary consumers
        
             
        
        
        
I think the answer to this is C
        
             
        
        
        
Human exploitation of fragile ecosystems can lead to the droughts and arid conditions characteristic of desertification. Effects include land degradation, soil erosion and sterility, and a loss of biodiversity, with huge economic costs for nations where deserts are growing.