John Cupp, CFA, has several hundred clients. The values of the portfolios Cupp manages are approximately normally distributed with a mean of $ 800000 and a standard deviation of $ 250000. The probability of a randomly selected portfolio being in excess of $1000000 is :（）
A. 0.3773.
B. 0.6227.
C. 0.2119.
参考答案：C
解析：
Although the number of clients is discrete, since there are several hundred of them, we can treat them as continuous. The selected random value is standardized (its z - value is calculated) by subtracting the mean from the selected value and dividing by the standard deviation. This results in a z - value of ( 1000000-800000)/250000=0.8. Looking up 0.8 in the z - value table yields 0.7881 as the probability that a random variable is to the left of the standardized value (i. e. less than $1000000). Accordingly, the probability of a random variable being to the right of the standardized value (i. e. greater than $1000000) is 1-0.7881=0.2119.