问题
单项选择题
（）the average age of the population increases, there are more and more old people to care for.
A. Unless
B. Until
C. As
D. While
答案
参考答案：C
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（）the average age of the population increases, there are more and more old people to care for.
A. Unless
B. Until
C. As
D. While
参考答案：C