定语从句：先行词是物不定代词与关系词
问题描述：
定语从句：先行词是物不定代词与关系词
新概念第二册Unit2 Lesson25练习题中有一道题：
He said something and I understood it.He said something ________I understood.
（a）who
（b）whose
（c）whom
（d）which
个人理解这应该是个定语从句,something是先行词,I understood是定从,关系词我选的是（d）.
但是语法书上和老师讲的时候,都说是物为不定代词,关系词用that,不用which.但是选项里却没有that.求解物为不定代词时关系词到底可不可以用which?
如果不可以,这里怎么解释；如果可以,那若以后遇见物为不定代词时,应该怎么选?which还是that?二者区别为何?
求赐教.
答
需要提醒的是,anything, something等后面的形容词从句通常都用that引导,但偶尔也可用which替代that.
如：We have proved that anything which exists is real./ If you have something to say which is on the subject, wait till you have a chance and say it to the whole class. 所以选D是对的.请问一下“偶尔”的情况是什么？在什么情况下which可以代替that？如果两个都出现，还是选that吗？上面一句是我在翻语法书的时候，上面这么解释的。这么着吧，有that优先选that,没that,只能选which.谢谢解答。