Passage Summary: A psychologist argues that because humans need personal interaction and you can't fulfill a 'thank you' obligation anonymously, the social benefit of gratitude requires the benefactor to know the source.

Conclusion: The social function of reinforcing beneficial behavior only works if the person being thanked knows who is expressing the gratitude.

Reasoning: The obligation to express gratitude cannot be fulfilled anonymously, and human psychology is primarily driven by personal interaction.

Analysis: This is a 'Role in Argument' question, so we need to see how the first sentence relates to the rest of the text. The first sentence is a claim that is not supported by any other statement in the passage; rather, it is used as a foundational premise to support the final 'Thus' statement. It establishes a necessary condition for fulfilling a specific obligation. Because it provides support for the conclusion without receiving support itself, it functions as a primary premise.