In the First$-$In$-$First$-$Out $($FIFO$)$ method, how are the costs of units in the opening inventory which are subsequently completed, treated?

Group of answer choices

They are combined with the costs of units begun with and completed in the current period.

They are averaged with the costs of all units in the inventory in the current period.

They are kept separate from the costs of the units begun with and completed in the current period.

They are excluded from the inventory valuation calculations in the current period.