How do you calculate the opportunity cost for: Mark can produce 50 baseballs in a month and Katie can produce 42 baseballs in a month. Also, Mark can produce 40 bats in a month and Katie can produce 30 bats in a month. What is Mark's opportunity cost of producing 10 bats? A) 15 baseballs B) 20 baseballs C) 45 baseballs D) 60 baseballs E)None of these