The proof _______ he killed the driver while robbing the taxi wa

The proof _______ he killed the driver while robbing the taxi was obtained by two passers-by.
A、that
B、what
C、which
D、why
【正确答案】：A
【题目解析】：先判断 he killed the driver while robbing the taxi 为从句部分；空格在从句内不做成分，因此该句不是定语从句而是同位语从句，因此选that。that在同位语从句中不充当任何成分，且不能省略。