单选题 Most of the older civilizations which flourished during the fifth century B.C. {{U}}are died out{{/U}}.

A、 they have died out
B、 has died out
C、 have died out
D、 they had died out

【正确答案】 C

【答案解析】[分析] 结构应用。本句是个复合句：Most of the older civilizations是主语，后面是which引导的定语从句which flourished during the fifth century B．C．因此划线部分应是谓语动词；根据主语，谓语应用复数形式；因此选项C正确。