This is an example of the economic concept of reciprocity.
Reciprocity is a mutual exchange of goods, actions, energy, time, emotion, etc. between two people. Reciprocity can and should be uniquely defined for each friend depending on his or her situation.
Here, in this situation, one decides to buy their friend lunch after she helped him study for his exam. So, this is an example of the economic concept of reciprocity. So here in this type of friendship, one is receiving from and giving to the other person, and this way the friendship is balanced.
Thus, when reciprocity is used in an honest, meaningful way with integrity, it can be incredibly powerful.
Hence, option C is correct.
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