Imagine that Reva has joined Enviropro, her neighborhood environmental-cleanup group. Enviropros objective is to maintain a clean neighborhood and provide better garbage collection facilities. Reva encourages her friend Marcel to join the group as well. Marcel refuses as he does not want to pay to join this group. He argues that he already gets collective benefits even without having paid an enrollment fee to join the group. In this scenario, Marcel's argument best exemplifies _______.