Tom and billy are playing a game. they both toss a coin simultaneously. if the results of the tosses are the same, both heads or both tails, tom wins. if the results of the tosses are different, one heads and one tails, billy wins. this is an example of

Zero-sum game

Explanation:

Zero-sum game is defined as the strategy in which an individual or team can become a winner only if the other individual or team loses in a competitive situation.The net sum of the situation is zero in this case.This strategy is from game theories.

According to the situation, Tom and Billy's winning or losing result will create a zero sum situation .If Billy win ,Tom will lose and if Tom wins,Billy will lose.Therefore the net of their win and lose will be zero.

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a would be the answer