Opportunity Costs calculation

How to calculate opportunity costs using PPC | Econ homework | I think it's Econ (https://www.youtube.com/watch?v=lJ9VzBiI-4o) explained the calculation of opportunity costs. In around 7 minutes, he changed the calculation direction from C - > D to D -> C. Why isn't the direction from D -> E appropriate to explain Opportunity cost? I simply thought the guitar's opportunity costs went to 0 because the firm used all opportunity costs for the piano, but it seems my understanding isn't correct.