For this chain-of-thought reasoning and answer, what was the question?
The relevant information to answer the above question is: The structure was renamed in 2009 by the Willis Group as part of its lease on a portion of the tower's space.
 A: 2009

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
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Q: The Willis Tower, built as and still commonly referred to as the Sears Tower, is a 110-story, 1,450-foot (442.1 m) skyscraper in Chicago, Illinois, United States. At completion in 1973, it surpassed the World Trade Center towers in New York to become the tallest building in the world, a title it held for nearly 25 years and remained the tallest building in the Western Hemisphere until 2014 and the completion of a new building at the World Trade Center site. The building is considered a seminal achievement for its architect Fazlur Rahman Khan. The Willis Tower is the second-tallest building in the United States and the Western hemisphere -- and the 16th-tallest in the world. More than one million people visit its observation deck each year, making it one of Chicago's most popular tourist destinations. The structure was renamed in 2009 by the Willis Group as part of its lease on a portion of the tower's space.

Based on this passage, when did the sears tower become the willis tower?