Card Problem answer (from the Historical Perspective entry)

The correct answer for the card problem is the set of cards "A'' and "5''. Almost everyone gets "A'' correctly. It is indeed quite obviously that if we turn it and find an odd number, the rule was proven wrong. "B'' can not do that, despite some people picking it, simply because nothing was said about what happens when the letter is a consonant. "2'' is a very interesting case. It can provide confirmation, but not proof, to the rule, if we find a vowel at the other side. But if it is a consonant, that means nothing. It is a wonderfully safe check for the rule, since it will not be proven wrong! Hence, a terrible check of the rule, a wrong answer. On the other hand, while "5'' is often neglected, if we find a vowel in the other side, the rule was indeed broken