What Do Economists Mean by the Phrase "Sunk Costs Are

Multiple Choice

What do economists mean by the phrase "sunk costs are sunk"?

A) Sunk costs are irretrievable,but they do lower profits and thus affect the firm's output level.
B) Sunk costs are a primary reason why marginal costs tend to increase.
C) Sunk costs cannot be recovered and are irrelevant to future decision making.
D) Sunk costs lower consumer welfare,because producers "pass on" these costs in the form of higher prices.