单项选择题

A．carbon-14
B．carbon-15
C．carbon-16
D．carbon-17
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单项选择题
A．low in educationB．low in incomeC．either very young or very oldD．all the above
A．low in education
B．low in income
C．either very young or very old
D．all the above
填空题
Alvin Toffler published a book named().
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