老师可以解释一下b选项吗 谢谢

问题如下：

The covariance of returns is positive when the returns on two assets tend to:

选项：

A.

have the same expected values.

B.

be above their expected value at different times.

C.

be on the same side of their expected value at the same time.

解释：

C is correct.

The covariance of returns is positive when the returns on both assets tend to be on the same side (above or below) their expected values at the same time, indicating an average positive relationship between returns.

当Xi、Yi均同时大于或者均小于X的期望、Y的期望时，COV(X,Y)是正的。