Answer:Primary reinforcers  
explanation reinforcers that have innate reinforcing qualities.
 
        
             
        
        
        
20.
They told me that my answer was too short so I'm just adding this on, don't mind me.                                    
        
             
        
        
        
THIS IS known as the PERICARDIUm
        
             
        
        
        
False biology is about learning the anatomy of plants and humans
        
             
        
        
        
Answer:
They digest without the use of teeth.
Explanation:
The food trickles from the crop into the bird's stomach (proventriculus or gizzard) where digestive enzymes are added to the mix and physical grinding of the food occurs. The gizzard is why chickens do not need teeth.