Professor Kwan studies violence and designs a study of the effects of video game violence on children. She recruits low-income, Hispanic children from schools near the university to participate. Each child is assigned to play either a violent or nonviolent video game 2 hours each evening for a month. The children's teachers are asked to assess changes in behavior. To assure good participation, the participants are offered a chance to win a family trip to a major theme park. Which of the Belmont Report principles is violated by this incentive?
a) beneficence
b) fidelity and responsibility
c) respect for persons
d) justice