Bart, Chang and Omar rode their bicycles to school. Each had a chain and a lock. Bart locked his bike to a lamppost and to Chang’s bike. Chang locked his bike to the lamppost and also to Omar’s bike. Omar locked his bike to the lamppost and to both Bart and Chang’s bikes. Each bike was now held by two locks.

Later that day Bart lost his key! With the two keys they had left, they could unlock only one of the bicycles. Whose bicycle was it?

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