<span>Active transport means</span><span> moving against a concentration gradient, from a lower concentration to a higher one</span> 
        
             
        
        
        
Answer:
they have lower levels of the neurotransmitter serotonin
 
        
             
        
        
        
The best and most correct answer among the choices provided by the question is the fourth choice "cenozoic"
The other two are the Mesozoic and Paleozoic Eras. TheCenozoic<span> spans only about 65 million years, from the end of the Cretaceous </span>Period<span> and the extinction of non-avian dinosaurs to the present. The </span>Cenozoic<span> is sometimes called the Age of Mammals, because the largest land animals have been mammals during that time.</span>
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