If a Customs Union Includes the Lowest-Cost World Producer of a Product

If a customs union includes the lowest-cost world producer of a product, then member countries:

A) will always be better off in trade with that product.
B) will always be worse off in trade with that product.
C) can be better off or worse off depending on the strengths of the trade diversion and trade creation effects for that product.
D) will no longer export or import that product.