115. Read the following passage, then answer the question below.

Imagine two suspects arrested for a crime and held in separate rooms. Each is offered the same deal: betray the other and walk free while the partner serves a long sentence, or stay silent. If both stay silent, each receives a short sentence on a minor charge. If both betray, each receives a moderately long sentence. Whatever the partner does, betraying gives a better result for the individual: if the partner stays silent, betrayal means freedom, and if the partner betrays, silence would mean the longest sentence of all. Rational suspects therefore betray, and both end up worse off than if each had kept quiet. This puzzle, formalized in 1950 by the mathematicians Merrill Flood and Melvin Dresher and named by Albert Tucker, is called the prisoner's dilemma.

The dilemma matters because it captures situations far from any police station. Two firms deciding whether to cut prices, or two countries deciding whether to build weapons, face a similar structure: each gains by acting selfishly, yet mutual selfishness harms both. In the real world, however, such encounters are rarely one-time events. In the early 1980s, the political scientist Robert Axelrod invited experts to submit programs to compete in a repeated version of the game. The winner was the simplest entry, called tit for tat, which cooperated on the first move and thereafter copied whatever the opponent had done on the previous move. Axelrod found that the successful strategies were nice, never being the first to betray; retaliatory, answering betrayal at once; and forgiving, returning to cooperation once the opponent did.

Critics point out that the tournaments' results depend on the particular mix of programs, and that human beings often cooperate for reasons the model leaves out, such as trust or fear of reputation. Still, the game remains a powerful tool, showing how cooperation can emerge among self-interested parties when they expect to meet again.

Which of the following describes tit for tat?