He wrote a letter ______ he explained what had happened in t

He wrote a letter ______ he explained what had happened in the accident.A.what B.which C.where D.how 选哪个,重要的是为什么.

He wrote a letter ___C___ he explained what had happened in the accident.
A.what B.which C.where D.how
因为从句中不缺名词成分,所以不是which
而根据句意来看是,应该是
He wrote a letter on which he explained what had happened in the accident.
on which=where
再问： 什么叫不缺名词成分？
再答： 你就理解为不缺主语和宾语
再问： which是充当主语的？
再答： which指的是letter 所以是on which =where
再问： in which 是where
再问： 你记错了
再答： 方位词+which都等于where,不要局限于一个
再问： 老师说 将先行词带入从句 然后可以知道从句缺少状语 这怎么看出来的？
再问： 是怎么带入的？
再答： 这个其实要看句意，他阐述了在事故中发生的事，在哪？在信上。。所以是缺状语
再问： 奥！！ 我懂了，万分感谢！！