At Ultrinsic.com, students pay a small entry fee to compete in grades-based contests for cash prizes. Suppose that25students from your economics class each pay $10to enter a grades-based contest. This would create a $250prize pool. An equal share of the $250pot is awarded at the end of the term to each contestant who earns an A in the course. If four students earn A's, each receives $62. If only one student earns an A, that person gets the entire $250pot. What economic concept does Ultrinsic.com use to encourage participants to learn more and get better grades?