Ben bakes bread and Shawna knits sweaters. Ben and Shawna both like to eat bread and wear sweaters. In which of the following cases is it impossible for both Ben and Shawna to benefit from trade?
a) Ben's bread is of higher quality than Shawna's sweater.
b) Shawna's sweater is of higher quality than Ben's bread.
c) Ben and Shawna have equal opportunity costs for producing bread and sweaters.
d) Ben can produce both bread and sweaters at a lower opportunity cost than Shawna.