i did alevel last year
opportunity cost of sth is what you sacrifice to get that thing.
for example,you can either go to have a english class or go shopping,
so the opportunity cost of ur english class is shopping,
because u have english class and can't go shopping at the same time
in the real world economy,firms make rational decesions based on valuing opportunity cost of each decesion
答案不是跟书上那样严密
但是这是我自己的理解 definition也是绝对正确的