He wrote a letter he explained what had happened in the accident. A. whatB. whichC. wher

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A. what
B. which
(答案→)C. where
D. how
解析：    
试题分析:句意：他写了一封信，在信里解释了事故当中所发生的事情。从结构判断是定语从句，从句中意思完整，不缺少成分，应该用关系副词，what、how不引导定语从句，故答案选C。