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Laura: Your encyclopedia is out of date.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.

问题： Which one of the following most accurately describes a reasoning error in Laura’s argument?


答案是C.
It mistakes something that is necessary for its conclusion to follow for something that ensures that the conclusion follows.
不好意思，我比较笨读了多次还是不能理解选项C的意思。而且，也没弄懂狒狒的介绍，特请教各位高手！
谢谢，请不吝赐教！

C选项的意思是：
Laura错误地将一个结论成立的必要条件当作了一个结论成立的充分条件。
具体到这道题来说
如果要说Fermat没有说谎或犯错误，那必须要现在有人能够证明出这个猜想（这是必要而非充分条件，也就是说即使现在有人证明除了Fermat的猜想，Fermat当时还是有可能说了谎或者在证明的时候犯了错误）

不胜感谢！解释的非常清楚！