<span>tubulin, the microtubule protein</span>
        
             
        
        
        
Answer: Smooth muscle
Explanation:
I got it wrong on usatestprep and when it showed me the answer key, smooth muscle was the correct answer.
 
        
             
        
        
        
Mastication is just a fancy way to say chewing. Since nothing chemical is going on and the food is just being chewed, the answer is physical. 
        
             
        
        
        
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