单项选择题

A person's calorie requirements vary ______ his life.

A．across
B．throughout
C．over
D．within
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单项选择题
Agriculture was a step in human progress ______ which subsequently there was not anything comparable until our own machine age.
A．in
B．for
C．to
D．from
单项选择题
The basic features of the communication process are identified in one question: Who says ______ through what channel to whom
A．how
B．when
C．what
D．such
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