问题如下：
4. Martinez’s Conclusion 1 is:
选项： correct.
incorrect because the mean and variance of WTI oil prices are not constant over time.
C.incorrect because the Durbin–Watson statistic of the AR(2) model is greater than 1.75.
解释：
B is correct. There are three requirements for a time series to be covariance stationary. First, the expected value of the time series must be constant and finite in all periods. Second, the variance of the time series must be constant and finite in all periods. Third, the covariance of the time series with itself for a fixed number of periods in the past or future must be constant and finite in all periods. Martinez concludes that the mean and variance of the time series of WTI oil prices are not constant over time. Therefore, the time series is not covariance stationary.
这题从Exhibit 2看出是有AR现象，是不是有没有AR现象和covariance-stationary 3个条件是否constant & finite没有关系？
但如果有ARCH现象和单位根，那么则不是 covariance-stationary。还是只有单位根会违背 covariance-stationary ？
这里学的有点混乱，谢谢
