短文改错（共10小题；每小题1分，满分10分）

Dear David,

I have recently felt guilty, stay sleepless at night. You may wonder how. To tell you the truth, it is just because of the factthatI seldom cheat in exams and I just can’t stop me. In many exams, I used cellphones to surf the net for answer. Besides, I sometimes referred themto my notebooks so copied something. As you know, cheating do much harm to our learning and self-confidence. It’s time that I take action to stop it. I would appreciate it you could offer me some advice.

Looking forward to your reply!

机体内维生素B2的辅酶存在（）.

A.FAD

B.NAD

C.CoASH

D.TPP