If a smooth curve is to represent a probab

问题：

If a smooth curve is to represent a probability density function, what two requirements must be satisfied The :（）

A. area under the curve must be zero and the curve must not fall below the horizontal axis.

B. area under the curve must be one and the curve must not rise above the horizontal axis.

C. total area under the curve must be one and the curve must not fall below the horizontal axis.