Why do sunk costs not affect rational decision-making?

Because sunk costs change with the level of output and thus should be ignored only when production is fixed.

Because sunk costs are past expenditures that cannot be recovered and therefore do not affect the incremental costs or benefits of future choices.

Because sunk costs represent part of the total cost and must always be included when comparing any alternatives.

Because sunk costs will be recovered if you continue the activity, so they determine the future returns and decisions.