He came back for the book ( ) he had forgotten.

问题描述:

He came back for the book ( ) he had forgotten.
括号里的空缺应该填以下哪个关系代词:who whome whose which that of which
of which
麻烦各位说明理由
还有参考答案好像是A

只能选that 这是同位语从句不是定语从句
后面那个句子是解释说明而不是修饰