Question

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

1. A.
   as
2. B.
   which
3. C.
   what
4. D.
   that

Answer 1

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.