Joseph: My encyclopedia says that the mathematician Pierre de Fermat died in 1665 without leaving behind any written proof for a theorem that he claimed nonetheless to have proved. Probably this alleged theorem simply cannot be proved, since---as the article points out---no one else has been able to prove it. Therefore it is likely that Fermat was either lying or else mistaken when he made his claim.
Laura: Your encyclopedia is out of date. Recently someone has in fact proved Fermat’s theorem. And since the theorem is provable, your claim---that Fermat was lying or mistaken---clearly is wrong.
50. Which one of the following most accurately describes a reasoning error in Laura’s argument?
(A) It purports to establish its conclusion by making a claim that, if true, would actually contradict that conclusion.
(B) It mistakenly assumes that the quality of a person’s character can legitimately be taken to guarantee the accuracy of the claims that person has made.
(C) It mistakes something that is necessary for its conclusion to follow for something that ensures that the conclusion follows.
(D) It uses the term “provable” without defining it.
(E) It fails to distinguish between a true claim that has mistakenly between believed to be false and a false claim that has mistakenly been believed to be true.
c说laura错在把必要条件当充分条件。看似有道理，而且其他答案也没有合适的，可我认为，joseph得出结论“Therefore it is likely that Fermat was either lying or else mistaken when he made his claim.”就是基于前面“since---as the article points out---no one else has been able to prove it”，laura指出假想已经被证明，那么也就是打断了joseph的前提，这样joseph的结论也就是错的了。这样分析的话，其实laura的argument就不是必要而不充分了。
