Because the old people about thirty percent of the population, we should care more about

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A. look up
B. fix up
(答案→)C. make up
D. end up
解析：look up“查阅”，fix up“修理”，make up“组成，占到”。end up“结束”根据题意，应该选make up。本句的意思是“因为老人占到人口的百分之三十，所以我们应该更关心他们。”所以本题选C。