Please help, I do not understand.

Concept: Opportunity Cost 2 a; Question Help Every society faces trade-offs because we live in a world of scarcity. Suppose a student-athlete has the opportunity to earn $600,000 next year playing for a minor league baseball team. $300,000 next year playing for a European professional football team, or $0 returning to college for another year. The opportunity cost of the student-athlete returning to college next year is $D. (Enter your response as an integer.)