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

【正确答案】 C

【答案解析】从句法结构上看，句中介词at需要宾语，系动词was需要主语，而能同时在主、从句中作两种句子成分的关系代词有what，whatever，whoever，whomever，whichever等。从这一点出发，可知本题正确答案为C，相当于the speed which。