Read this and answer the question. If the question is unanswerable, say "unanswerable".

As mayor, Young sought to revive the city by seeking to increase investment in the city's declining downtown. The Renaissance Center, a mixed-use office and retail complex, opened in 1977. This group of skyscrapers was an attempt to keep businesses in downtown. Young also gave city support to other large developments to attract middle and upper-class residents back to the city. Despite the Renaissance Center and other projects, the downtown area continued to lose businesses to the suburbs. Major stores and hotels closed and many large office buildings went vacant. Young was criticized for being too focused on downtown development and not doing enough to lower the city's high crime rate and improve city services.

What group of buildings opened in 1977?
The Renaissance Center