单项选择题

The main reason for the population growth isn't so much a rise in birth rates ______ a fall in death rates as a result of improvements in medical care.

A. and
B. but
C. or
D. as
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______ nearly all behavior is learned behavior is a basic assumption (假设) of social scientists.
A. /
B. That
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单项选择题
______ time went on, the teacher's words proved true.
A. As
B. While
C. With
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