单选题 ______ is that I promised to make friends with the dishonest girl.
A. The only thing what I regret B. What I regret most
C. All what I really regret most D. That I regret most

【正确答案】 B

【答案解析】[解析] 最让我后悔的是答应与那个不诚实的女孩交朋友。本题可以采用排除法。选项A，选项C里面都包含了一个定语从句，而what不能引导定语从句。如果选D项，主句将变成That is that…。所以只能选B项，这是what引导的主语从句。