单项选择题

This line was carrying equal number of eastbound and westbound trains, and they ______ regularly.

A．altered
B．alternated
C．switched
D．exchanged
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单项选择题
It was believed that his death ______ with the robbery of the bank downtown.
A．accompanied
B．coincided
C．correlated
D．conformed
单项选择题
College students in this city have set up the Cleaner Air Society to help urban citizens become aware of the to our environment.
A．conditions
B．situations
C．dangers
D．threats
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