单项选择题

Differences in positions adopted by oxygen and hydrogen atoms account for variations in the crystalline structure of different forms of ice.

A．are caused by
B．explain
C．derive from
D．constitute
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单项选择题
Cowries shells were once in widespread use as a token of value.
A．a symbol
B．an amount
C．a thing
D．an investment
单项选择题
Chimpanzees are frequently used as stand-ins for human beings in experiment.
A．partners
B．role-models
C．stand-bys
D．substitutes
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