An organization runs a program where volunteers travel to a remote village to help build a school. Three volunteers participate:

• Volunteer A is a wealthy individual who pays for their own travel and expenses. They work long hours on the construction site. Upon returning, they never mention their participation to anyone and request that the organization not publicize their involvement.
• Volunteer B is a university student who receives a grant to cover all travel and living costs. Participating in the program fulfills a mandatory community service requirement for their degree. They document their experience extensively on social media to build a personal brand as a humanitarian.
• Volunteer C is a local resident from a neighboring town. They spend their weekends helping with the construction, using their own tools and forgoing paid work they could have done instead. Their primary stated reason is to ensure the children in the next village have the same educational opportunities as their own.

Based on the provided descriptions, which volunteer's actions most closely align with the economic definition of a purely altruistic act? Justify your answer by analyzing the costs borne and the potential benefits received by each volunteer.