1. The problem asks which costs Carolyn should consider when calculating the opportunity cost of leaving college. 2. Opportunity cost includes the value of the next best alternative foregone. 3. Costs already incurred (sunk costs) like 90 credit hours completed and tuition already paid should be ignored because they cannot be recovered. 4. The time spent working on the app instead of studying is a current and future cost and should be included. 5. Skills gained from the final year of courses represent potential benefits lost if she leaves, so they should also be considered. 6. Therefore, Carolyn should ignore sunk costs (credit hours and tuition) and consider the time spent on the app and skills lost. Final answer: Carolyn should consider the time she will spend working on the app instead of studying and the skills she may gain from her final year of economics courses when calculating opportunity costs.