在Chapter 2 The Law of Comparative Advantage这一章里，

    If nation A can produce 5 units of good X or 10 units of good Y and nation B can produce 4 units of good X or 12 units of good Y we can conclude that both nations would gain from trade if nation A sold _____ units of good _____ for one unit of good _____

a. 0.4; Y; X
b. 2.5; Y; X
c. 2.5; X; Y
d. 0.4; X; Y

我的当时的想法是A国集中生产产品X，B国集中生产产品Y。然后A用5X去交换B的12Y，这相当于说用(5/12)X换1Y。但答案选C，为什么