An individual's production possibilities are represented by a feasible frontier showing the trade-off between hours of free time and bushels of grain produced. Moving from a situation with 16 hours of free time and 50 bushels of grain (Point A) to one with 12 hours of free time and 60 bushels of grain (Point B) is a choice along this frontier. Based on this information, what is the opportunity cost of producing the additional 10 bushels of grain?