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

答案及解析： C

解析：

题干的介词at后为一宾语从句，该宾语从句的主语是一主语从句，连接代词what既引导主语从句，又在从句中作thought的宾语.连词that虽然也能引导主语从句，但它不能在从句中充当成分，故答案为C.