单项选择题

Doctor Stevenson is analyzing one element______.

A．carbon-14
B．carbon-15
C．carbon-16
D．carbon-17
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单项选择题
What did the students find when they came to the river this year
A．Less garbage.
B．More garbage.
C．More waterbirds.
D．More swimmers.
填空题
Alvin Toffler published a book named
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