Structure breaks需要掌握吗

问题如下：

Describe the four features of a time series that violate the assumptions of covariance stationarity.

选项：

解释：

a. Random walks—The variance of the process changes over time because a shock is never “forgotten” by the process.

b. Deterministic trends—The mean of the process is changing over time due to a time trend; for example, the series has a positive growth rate.

c. Deterministic seasonality—The mean of the process depends on the time observation due to a seasonal, repeating pattern.

d. Structure breaks—The model parameters change over time.