问题补充：
--- I didn’t believe you made such a mistake.
---Oh, I’m sorry. I did it . I’m serious.
A. on purpose
B. by accident
C. by the way
D. as for
答案：
A. on purpose
(答案→)B. by accident
C. by the way
D. as for
解析：短语辨析. A.故意地 B.意外地 C.顺便说说 D.关于,至于.根据题意,我不相信你犯了这样一个错误.——哦，对不起。我是意外做的。我是认真的。选B。
