为什么因变量也要检测是否协方差平稳？

问题如下：

DeMolay cautions Kamini: “Remember that when we analyze two time series in regression analysis, we need to ensure that
(1) neither the dependent variable series nor the independent variable series has a unit root, or (2) that both series have a unit root and are not cointegrated.
Unless Condition 1 or Condition 2 holds, we cannot rely on the validity of the estimated regression coefficients.

DeMolay's caution given in Condition 1 is best described as:

选项：

incorrect because only the dependent variable series needs to be tested for the absence of a unit root.

incorrect because only the independent variable series needs to be tested for the absence of a unit root.

correct

解释：

When working with two time series in a regression analysis, both of the series must be tested for the presence of a unit root. If neither series has a unit root, you can safely use linear regression to test the relationship between the two time series.