问题如下：
An economic analyst has calculated the probabilities of three possible states for the economy next year:growth,normal and recession.Which of the following would an analyst use to estimate the expected value of the stock index under three scenarios?
选项：
A.Multiplication rule.
B.Addition rule.
C.The total probability rule.
解释：
C is correct. When the scenarios are mutually exclusive and exhaustive,we can analyze the event using the total probability rule.
加法法则和乘法法则也有专门针对互斥事件的公式，为什么不能用这两个呢？
