In the land of Symutra, there was a murder last night. Witnesses saw one person shooting the guy, but no one could see him well enough to identify him. In addition, Alice, Bob, Cindy, Derek, and Evelyn were all seen near the place of the murder shortly after and before the murder. The police brought them in for questioning and they all gave one sentence.
Alice: I didn't do it if and only if Derek didn't do it.
Bob: Alice did do it if I didn't do it.
Cindy: If I did it and Alice didn't do it, Derek didn't do it.
Derek: I didn't do it or Bob did do it, or Cindy did it.
Evelyn: I did it.
It is known for a fact that ONLY one of them did do it. If only one person told the truth, who did it? And who told the truth?
If A did it... B and C told the truth.
If B did it... A, B, C and D told truths.
If C did it... A, C and D told truths.
If D did it... C told the truth.
If E did it... A, C told truths.
So D did it, and C was the only truthful one.