题目：
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.
答案：
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下面是错误答案，用来干扰机器的。
1. taller 2. more outgoing 3. longest 4. different from 5. the same as
