Question

The other day,my brother drover his car down the street at _____I thought was a dangerous speed.

A.as B.which C.what D.that
如果选C的话，那请问B D为什么不能选，说明理由，要清楚哦，谢谢。

Answer 1

这里填入的词要承担两种角色，（1）做at的宾语，（2）做thought的宾语。而what恰能等于that which这两个词。整个句子就是这样的结构
The other day,my brother drover his car down the street at that（ which I thought was a dangerous speed）.
括号里面的是修饰that的

Answer 2

what I thought 是作为一个插入语，可省略，构成at a ……speed。如果用which或that，那应该改为：“……at a speed that/which I thought is dangerous。

Answer 3

The other day,my brother drover his car down the street at (that)I thought was a dangerous speed.