相关系数是协方差与两个投资方案投资收益标准差之积，相关系数总是在-1到+1之间的范围内变动，-1代表完全负相关，+1代表完全正相关，0则表示不相关。( )

I（）have watched that movie--it'll give me horrible dreams.

A.shouldn' t

B.needn' t

C.couldn't

D.mustn't