A risk manager at a leading global bank is conducting a time series analysis on equity returns. The goal of the analysis is to determine whether the equity returns time series exhibits covariance stationarity. Covariance stationarity is a fundamental property in time series analysis that ensures consistency in the mean, variance, and autocovariance over time, making statistical inferences and model predictions more reliable. Which of the following statements describes a necessary criterion for a time series to be classified as covariance stationary?