Four people are travelling sustainably to an event by train. Alan, Brian, Cathy, and Debbie discuss all the green choices they make to help the environment. But when they get up to disembark, a guilty secret is revealed: One of the four has left behind a disposable coffee cup.

Alan says: "It wasn't me. Cathy was drinking the coffee."   Brian says: "It wasn't me. Debbie did it!"
Cathy says: "It wasn't me. I don't drink coffee!"   Debbie says: "It wasn't me. It was one of the men."

The statements contradict each other. But if you know there are four true statements and four false ones, you can work out the one person responsible. Who left the cup?