问题如下：
3. Based on Exhibit 1, the correlation between the debt ratio and the short interest ratio is closest to:
选项：
A.−0.3054.
B.0.0933.
C.0.3054.
解释：
A is correct.
The correlation coefficient equals the covariance between
variables X and Y divided by the product of the standard deviations of
variables X and Y, as follows:
[#PZMATH1332#]
老师能给一下standard deviation的公式吗？一级学的忘了。standard deviation 永远是正数吗？
