问题: Accordingtothesecondparagraph,whendidcellphonesactuallyhelptoreducepedestrianandtrafficfatalities?
A: Rightaftercellphoneswereinvented.
B:eforethenumberofcellphoneusersreachedacriticalmass
C:Whencellphoneuserstotaledtoacertainnumber.
D:Whenthenumberofcellphonesdecreasedtoacertainnumber.
