NO.PZ2020010302000003
问题如下：
What two properties must a PMF satisfy?
选项：
解释：
A PMF must be non-negative and sum across the support of the random variable to 1.
two defining properties of probability：0<=P(E)<=1; P(E1)+P(E2)+......+P(En)=1,if E1、E2...En,is mutually and exhaustive.
properties of PMF:non-negative;the sum across all value in the support of a random variable must be one.
一个是概率的性质，一个函数的性质，但好像一样，概率之和等于1的两个条件，函数不需要吗？
