A) a common ancestor 
Homologous structures are structures that are similar in related organisms. There is a basic pattern of bones which can be inherited from a common ancestor
        
             
        
        
        
That act will  help the sustainability of out environment because : C. It will slow down the degradation of the ozone layer
The usage of CFC will crate chlorine atoms that catalyze ozone degradation. By prohibiting this product, Government will slow down Ozone's degradation
hope this helps
        
             
        
        
        
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From what I remember in my 9th grade biology class, it's usually half that amount.
 
        
             
        
        
        
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An oil spill, fertilizer run off, and another thing i could think of is a flood hope this helped :)
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