In a study, students agreed to fill out a short questionnaire for $1.50. At the end of the survey, respondents were offered a chance to exchange their $1.50 for a gift. Some students could exchange the money for a metal zebra pen, and some students could exchange the money for their choice of a metal zebra pen or two plastic pilot pens. Among the students who had two options for exchange, 53 percent chose to keep their money. Of the students who had only one option, 25 percent chose to keep their money. Explain these differences in the observed (average) willingness to exchange.