Answer:
the true cost (opportunity cost) of going trekking is what the amount that you could have earned from your part time job on that day instead of going trekking. If you were planning to to study, then the true cost of trekking with your friend would be the loss of all the potential knowledge that you could have gain from studying.
Step-by-step explanation:
opportunity cost is the loss of potential gain from other alternatives when one alternative is chosen.