1.I doubt it.

　　2.It can' t be/ true.不可能。

　　3.How is that possible? 那怎么可能呢?

　　4.It' s hard to believe.真难以相信。

　　5.Oh. Really?真的吗?

　　6.I don' t buy it./I can't believe it .我才不信。

　　7.I' m doubtful about that.我对此表示质疑。

　　8.You must be kidding!