It achieved peace because two world powers connected in a peaceful way. This created a sense of calmness for the end of the Cold War
The opportunity cost in this scenario is the three lost opportunities Harry experiences by deciding to go to his parents house. The term opportunity cost refers to <em>the loss of potential gain from other alternatives when one alternative is chosen. </em>The potential gain Harry may have lost by choosing to go to his parents for dinner instead could be <em>relaxation while fishing, His house painting being finished, and time spent with his friends at the birthday party. </em>These all can be considers lost opportunity due to choosing an alternate opportunity, that being dinner at his parents.
Had a social structure similar to Egypt's pyramid structure
Probably rivers and water sources, because you need water to farm, and the water by rivers is rich and the soil is good. So yea the rivers and other water sources would be the main thing.