Read the text and complete it with the right prepositions.
Choose the correct answer.
Why it's worth talking to each other
The twentieth century was the century of mass murder. Two World Wars, which most people are ashamed [of|for|in|off] today, took place within a period of thirty years. During this period, John von Neumann, a mathematician, and Oskar Morgenstern, an economist, came [for|upon|over|up] with their famous Game Theory. This theory examines the situations when people argue [on|about|for|to] something and the result for all participants depends on decisions made by others — as in negotiations. Game Theory is based on the observation that people in conflicts behave in the same way as they would when playing a board game. How do you win but follow the rules? The solution is simple: by approaching and talking to each other. Or, in the words of game theorists, by cooperating.
For example, most adults decide not to give each other Christmas presents, especially if they are very close [for|about|to|in] each other. But when people come [across|in|down|over] something beautiful at Christmas, still do buy it as a present. The person who gets the gift but didn't give one in return now has a bad conscience; the person who gave but didn't receive is disappointed. The result: conflict.
There are only two logical solutions and both have to do with communication: you have to decide whether (a) everyone or (b) nobody gets a gift. But this super-simple Game Theory solution can only be achieved if both sides go along with the plan and if they make an agreement to stick to it.