Crispin is asked by his daughter's third-grade teacher if he would be willing to act as a volunteer lunch monitor every day for the upcoming academic year. Crispin politely declines, but the teacher then asks if he would be willing to read to the class three days a week, every other week, for the next 6 months. Crispin agrees to this task. Which persuasion strategy has the teacher effectively used to gain Crispin's compliance?