问题如下：
A discrete random variable X follows the probability distribution in the table below:
Accoring to the above table, P (X=4) is equal to ?
选项：
A. 0.15.
B. 0.17.
C. 0.86.
解释：
B is correct.
Read the table.
所以这里p就是某个自变量发生的概率 f才是cumulative probability 是吗？