单项选择题

People are encouraged to use overhead bridges mainly because ______.

A．zebra crossing can be ignored
B．it is necessary to reduce traffic jam
C．they prevent accidents and loss of life
D．traffic is very heavy in large cities
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单项选择题
According to the passage, compared to 3G mobile phones, the second-generation phones ______.
A．do not receive signals transmitted with an electromagnetic field
B．can receive signals from a larger area than 3G mobile phone
C．cause worse impact when they are used close to the head
D．have no negative impact on human health
单项选择题
The double-blind laboratory tests means ______.
A．nobody taking part in the test knows if the 3G base station is transmitting signals
B．only the subjects in the test know if the 3G base station is transmitting signals
C．no researchers know if the 3G base station is transmitting signals
D．everyone in the test knows if the 3G base station is transmitting signals
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