John and Mary work at a bakery. John can decorate either 10 ice cream cakes or 4 wedding cakes; Mary can decorate either 8 ice cream cakes or 2 wedding cakes. According to this scenario, what is John's opportunity cost of decorating one wedding cake?
a) 2.5 ice cream cakes
b) 2 ice cream cakes
c) 4 ice cream cakes
d) 8 ice cream cakes