这道题的公式又是在哪里出现了？

问题如下：

The mean monthly return and the standard deviation for three industry sectors are shown in the following exhibit:

Based on the coefficient of variation, the riskiest sector is:

选项：

解释：

B is correct. The coefficient of variation (CV) is the ratio of the standard deviation to the mean, where a higher CV implies greater risk per unit of return：

$C{V_{UTIL}} = \frac{s}{{\bar X}} = \frac{{1.23\% }}{{2.10\% }} = 0.59$

$C{V_{MATR}} = \frac{s}{{\bar X}} = \frac{{1.35\% }}{{1.25\% }} = 1.08$

$C{V_{INDU}} = \frac{s}{{\bar X}} = \frac{{1.52\% }}{{3.01\% }} = 0.51$