根据上句完成下句 The reason why the boy was often late for school is still unknown
问题描述：
根据上句完成下句 The reason why the boy was often late for school is still unknown
The reason( )( )the boy was often late for school is still unknown
答
填 for which.定语从句中介词+关系代词可以相当于一个关系副词.关系副词why指原因.介词for也可以表原因,所以why相当于for which.