The other day,my brother drove his car down the street at __

这题的答案不填WHICH.
正确答案是:WHAT.
The other day,my brother drove his car down the street at _what___ I thought was a dangerous speed.
在这个句子中,at 是一个介词,后面要跟一个宾语从句,I thought是一个插入语,可以去掉,那么这个宾语从句本身要缺一个主语,作双重成分所以只能用WHAT了.
整个句子的意思是:
那天,我的兄弟以我认为是一个危险的速度沿着这条街开车.

i thought my brother drove his car down the street at a dangerous speed(which)

speed是先行词，修饰开车速度

地点名词 （先行词）+介词+which
先行词 the street