单选题
According to the second paragraph, when did cell phones actually help to reduce pedestrian and traffic fatalities?
A、
Right after cell phones were invented.
B、
Before the number of cell phone users reached a critical mass
C、
When cell phone users totaled to a certain number.
D、
When the number of cell phones decreased to a certain number.
【正确答案】
B
【答案解析】
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