______ caused the accident is still a complete mystery.

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问题 ______ caused the accident is still a complete mystery.

选项 A、How
B、That
C、Where
D、What

答案D

解析本题中要填的词应在从句中作主语，同时引导主语从句。既在从句中担任名词性成分又引导从句的只能是关系代词what；that是从属连词，不在从句中担任成分。其它两个是连接副词，主要作句子的状语。正确答案为[D]。

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