请问关于assumption题的做法

如果用充分必要条件来反推的话，是不是像以下的做法：

设原文结论是C，选项假设为A，然后对A取非，如果能推出C取非，那就是正确答案？

我一直没搞明白，假设是结论的什么条件？充分还是必要？

谢谢指教

Exports of United States wood pulp will rise considerably during this year. The reason for the rise is that the falling value of the dollar will make it cheaper for paper manufacturers in Japan and Western Europe to buy American wood pulp than to get it from any other source.

Which of the following is an assumption made in drawing the conclusion above?

(A) Factory output of paper products in Japan and Western Europe will increase sharply during this year.

(B) The quality of the wood pulp produced in the United States would be adequate for the purposes of Japanese and Western European paper manufacturers.

(C) Paper manufacturers in Japan and Western Europe would prefer to use wood pulp produced in the United States if cost were not a factor.

(D) Demand for paper products made in Japan and Western Europe will not increase sharply during this year.（B）

(E) Production of wood pulp by United States companies will not increase sharply during this year.

答案是B，C显然是错的。但是如何运用以上方法来证明它是错的？

C選取非 會推出原題結論而無法使原題結論不成立

m in J & E would prefer to ... if cost were a factor

C取非   Paper manufacturers in Japan and Western Europe would not prefer to use wood pulp produced in the United States if cost were not a factor.

价格不是考虑因素的话 欧洲用户不倾向于用美国wood pulp

但是原题干谈的是考虑价格因素的情况  所以C取非以后与题干谈的根本不是一个范畴 所以无法消弱原文  所以C 错