A sunk cost is money already spent that cannot be changed by the current decision, so it should not control which alternative is selected. In FE-style comparisons, you focus on differences in future costs and benefits across choices, not on past expenditures that are common and unrecoverable. Keeping sunk costs out of the analysis prevents biased choices and supports consistent economic ranking of alternatives (FE Handbook 10.6, Engineering Economics, p. 236–237; Civil CBT Exam Specifications, p. 482)