Chris promises Dina $40,000 if she graduates from Eagle College. Dina enrolls in Eagle, attends full-time for four years, and graduates. When Dina asks Chris for $40,000, Chris says, "I don't remember promising you $40,000. But if there was a promise, it's not enforceable, because we didn't bargain for it. And even if there was a promise that would otherwise be enforceable, I revoke it now." Can Dina enforce Chris's "promise"? Why or why not?