Unauthorized acces
Four friends have been identified as suspects for an unauthorized access into a computer system, of whom exactly one has committed the crime. They have made statements to the investigating authorities.
Alice said, “Carlos did it.”
John said, “I did not do it.”
Carlos said, “Diana did it.”
Diana said, “Carlos lied when he said that I did it.”
If the authorities also know that exactly one of the four suspects is telling the truth, then who did it?
1) Alice
2) Carlos
3) Diana
4) John
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Carlos & Diana were being contradictory, so either one of them must have told the truth and nobody else would or could by what the authorities found out. Alice and John were definitely lying, so with John's self-claiming innocence, it's obvious he's our guilty criminal.