I think it is C, because if I'm right present principal is something that is happening presently? If so, C would be the answer because they "are doing" the exercises right now in context.
        
             
        
        
        
The third choice is correct.
I hope this helps! :)
        
                    
             
        
        
        
Answer: Tia likes mustard, not ketchup on her burger.
Explanation:
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The correct answer is A. lunches and river.
A common noun is concrete - it refers to something you can touch, feel, something that surrounds you; words such as desk, mirror, rug, are all common nouns. 
B is incorrect because those are proper nouns, meaning they refer to the names of people or places. C is incorrect because eat is a verb, and side is an abstract noun. D is incorrect because both words are verbs.