Suppose Mark can produce 1 tennis racket or 3 baseballs in an hour. He works for 6 hours in a day and allocates his time in making these two goods. The different combinations of the goods that Mark can produce in 6 hours are shown in the following figure. What is Mark's opportunity cost of producing tennis rackets in terms of baseballs? Number of baseballs 18 15 12 9 6 3 0 1 2 3 4 5 6 7 Number of tennis rackets