Compared with Overhead bridges, zebra crossing is ______.

A．the impact of mobile phone signals on health
B．the negative impact of current signals on health
C．the long-term effects of mobile phones on health
D．the possible causes of the impact of mobile phones

A．3G signals and current signals transfer data at the same speed
B．3G mobile phone service will not be allowed for its bad impact
C．more tests would be done to check the results of the study
D．second-generation mobile phones will take the place of 3G mobile phone