In Artland the Opportunity Cost of Producing 1 Ton of Wheat

In Artland the opportunity cost of producing 1 ton of wheat is 2 tons of corn,while in Scienceland,the opportunity cost of producing 2 tons of wheat is 3 tons of corn.Both countries would be better off if:

A) Artland sells 4 tons of wheat to Scienceland in return for 7 tons of corn.
B) Artland sells 4 tons of wheat to Scienceland in return for 10 tons of corn.
C) Scienceland sells 4 tons of wheat to Artland in return for 7 tons of corn.
D) Scienceland sells 4 tons of wheat to Artland in return for 4 tons of corn.