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
Recent studies have identified four major global environmental risks— acid rain, ozone depletion, deforestation and the greenhouse effect.
A．recognized
B．proved
C．got rid of
D．multiplied
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