Evelyn Millie, a university psychology professor, says to her students, "All of you have worked hard, and if you continue to perform at a high level, I will pay for'J6' a pizza party for the class at the end of the year-if I think it is warranted." The students insist that they continued to "work hard," and they claim that the class grades "are high," but Millie gives no party. Can the students enforce Millie's promise?