题目：
A random variable that has a countable number of possible values is called a:（）
A. discrete random variable.
B. continuous random variable.
C. probability distribution.
答案：
参考答案：A
解析：
A discrete random variable is one for which the number of possible outcomes are countable, and for each possible outcome, there is a measurable and positive probability. A continuous random variable is one for which the number of outcomes is not countable.
