【题目】He wrote a letter ______ he explained what had happened in the accident.
A.what
B.which
C.where
D.how

【答案】C
【解析】句意：他写了一封信，在信中他解释了事故中发生的一切。a letter是先行词，从句本身主(he )--谓(explains )--宾(what had happened in the accident)成分齐全，缺少的是地点状语。可知使用关系副词where.故选C。