Answer:
The answer is C.
Explanation:
It is tangible boundaries (C) because these are natural/physical limitations. The other answers would mean that they were marked or created by humans. 
 
        
                    
             
        
        
        
Bill of rights have ammendments, declaration does not
        
             
        
        
        
African-American citizens made up a full three-quarters of regular bus riders, causing the boycott to have a strong economic impact on the public transportation system and on the city of Montgomery as a whole.