1. The problem asks: What are sunk costs? 2. Sunk costs are costs that have already been incurred and cannot be recovered. 3. Important rule: Since sunk costs cannot be changed by any current or future decisions, they are considered irrelevant for decision-making. 4. Therefore, sunk costs are never relevant when making decisions. 5. Explanation: Because sunk costs cannot be altered, focusing on them can lead to poor decisions. Instead, decisions should be based on future costs and benefits. Final answer: **Never relevant**.