In this task, you're given an open-domain question. Your task is to provide an answer to the given question. There is only one unique answer for each question. Your answer should be short, and refer to an entity, date, number, etc. Do not put your answer in the context of a sentence.
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Question: When did leningrad changed back to st petersburg?

Answer: 6 September 1991


Question: How many series are there of the mentalist?

Answer: 7


Question: Who did the vocals on mcdonald's jingle i'm loving it?

Answer:
Justin Timberlake