The other day my brother drove his car down the street at I thought was a dangerous spee

问题补充：

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

A. as

B. which

C. what

D. that

答案：

A. as

B. which

(答案→)C. what

D. that

解析：考查名词性从句。介词at后为宾语从句，I thought为插入语，从句中缺少主语，所以用what，故选C。