Both
It explains: the slowdown
        
                    
             
        
        
        
Explanation:
Please where is the question
 
        
             
        
        
        
Answer: The malicious boy
Explanation: The subject is the naming part. It tells you WHO the sentenced is about. When you read the sentence, ask yourself "WHO is this sentence about?" The person or thing the sentence is about the SUBJECT.
 
        
             
        
        
        
All would seem appropriate, but I'd say D, since it assists you in gathering data to support your clause