Answer:
oxygen enters blood cells by 'diffusion'
 
        
             
        
        
        
Answer:
The Answer is C
Explanation:
When scientists breed cows to produce more milk, this has not occured naturally and thus is not natural selection.
 
        
                    
             
        
        
        
Answer:
The ventricles of the heart have thicker muscular walls than the atria. This is because blood is pumped out of the heart at greater pressure from these chambers compared to the atria.
Explanation:
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