The answer is free enterprise
They kept all of their knowledge written down and their language is still here today to decipher what they have said.
Opportunity cost is a concept in economics and it refers to the cost of something that has to be given up to enjoy something better. This can be for example the benefits of second best alternatives (when the first best is chosen) or alternative use of something, which is not decided on (the cost of not using land for farming and using it for building a house instead).
