Answer:
So they are lighter and move quicker. 
Explanation:
With the small wings and bodies birds have it would be hard for them to escape predators and lift their own weights if their bones were dense like humans' bones. The hollow part of the birds' bone is connected to their lungs so they have enough oxygen to fly easily. Hope this can help you! :)
 
        
             
        
        
        
Rivers are large bodies contained of water which always move from the higher part to the lower. Over years It could change landscape of a land through erosion by slowly scrapping the land it passed through
        
                    
             
        
        
        
The cytoplasm is responsible for breaking down gluten. it is the first step in cellular respiration.
 
        
                    
             
        
        
        
1) A. Located in an enzyme
2) C. Synthesis of polymers from monomers
You're all good!