单项选择题

There have been UFO reports for ______.

A．15 years
B．50 years
C．six decades
D．four decades
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单项选择题
The US border with Canada is the ______ in the world.
A．longest defended border
B．longest undefended border
C．longest undefined border
D．longest coastal border
单项选择题
In which countries are there programs of persuing UFO A. China and US
A．
B．Chile and France.
C．France and Canada.
D．Egypt and Greece.
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