【单选题】Why does the author mention doorbell rings 
                    
A.It is a type of information.
B.It is a type of break.
C.It is a type of incentive.
D.It is a type of warnin
                
							【单选题】Why does the author mention doorbell rings 
                    
A.It is a type of information.
B.It is a type of break.
C.It is a type of incentive.
D.It is a type of warnin
                
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