Which of the following statements about time series cross$-$validation are correct? Select all that apply.

a$.$

In time series cross$-$validation, we use a series of test sets each consisting of a single observation.

b$.$

Forecast accuracy is determined by taking the average over the series of test sets.

c$.$

Time series cross$-$validation always uses one$-$step ahead forecasts.

d$.$

Time series cross$-$validation is equivalent to the traditional approach of dividing data in training and test set.