Assume point A on a linear production possibilities curve represents the combination of 12 coffees and 3 cappuccinos, and point B represents 3 coffees and 6 cappuccinos. Suppose coffees are on the vertical axis and cappuccinos are on the horizontal axis.

Assume point A on a linear production possibilities curve represents the combination of 12 coffees and 3 cappuccinos, and point B represents 3 coffees and 6 cappuccinos. Suppose coffees are on the vertical axis and cappuccinos are on the horizontal axis. The opportunity cost of a cup of coffee is:
A.3 cappuccinos.
B.1/3 of a cappuccino.
C.6 cappuccinos.
D.9 cappuccinos.
正确答案：1/3 of a cappuccino.