NO.PZ2021061603000034
问题如下：
Consider two variables, A and B. If variable A has a mean of-0.56, variable B has a mean of 0.23, and the covariance between the two variables is positive, the correlation between these two variables is:
选项：
A.negative. B.zero. C.positive.解释：
C is correct.
The correlation coefficient is positive because the covariance is positive. The fact that one or both variables have a negative mean does not affect the sign of the correlation coefficient.
correlation =Sxy/(Sx*Sy),然后题目说的covariance 方差永远都是positive ,题目强调positive 有什么意义呢？所以Sx和Sy有可能一正一负啊。是怎么退出正确答案的？
