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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In 1984, President Ronald Reagan proposed that the United States construct a launch laboratory for a new era of space exploration.
A．probing
B．revenge
C．invasion
D．expansion
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Recent research into aging suggests that the body's defense mechanisms may lose the ability to distinguish what is alien.
A．insane
B．infectious
C．foreign
D．poisonous
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