Which One of the Following Is Not a Step in Cross-Validation

Multiple Choice

Which one of the following is not a step in cross-validation with time series?

A) Use both the training and validation set to re-estimate the preferred model.
B) Split data series into early and later periods, representing both training and validation set.
C) Determine the proper forecast model based on R² results.
D) Explore suitable forecasting models to compute MSE, MAD, and MAPE.