When was the Willis Tower built?
1970
1973
1973
1973
1973
1973
1973
1973
1973
1973
1973
1973
1973
1973
1974
1973
1974
1973
1974
1973
1969
1974
1973
1969
1974
1973
1978
1969
1974
1973
1978
1977
1969
1974
1973
1978
1977
1969
1974
1978
1977
1969
1974
1978
1977
1969
1974
1973-76
1978
1977
1969
1973-74
1972
1974
1975
1978
1977
1969
1972
1974
1975
1978
1977
1971
1969
00-00
1972
1974
1973-76
1975
1978
1977
1971
1969
1983
1973-74
1972
1974
1975
1978
1977
1971
1969
1976
1983
1972
1974
2009
1975
1978
1977
1971
1969
1978-79
1976
1983
1981
1970
00-00
1973-80
1972
1973-79
1974
2009
1973-76
1975
1978
1977
1971
1969
1976
1983
1981
1968
1970
1st floor
1973-74
1984
1972
1974
2009
1979
1975
1978
7 years ago
1977
1971
1969
1 July 1973
1976
1983
007
1981
1968
1970
1984
1972
1974
2009
1979
1975
1978
1977
1971
1969
1964
7 years ago
1976
1983
1981
1968
1970
1st floor
1984
1972
1974
2009
1979
1973-76
1975
1978
1 May 1973
1973.
007
1977
1971
00 00
1969
00-00
1964
1976
1st century
1983
1981
1968
1970
1984
1972
1974
2009
1979
1975
1978
7 years ago
1 August 1973
00-05
1st year
1977
1971
1973-74
1969
1 July 1973
1964
79-80
1976
1983
007
1981
1968
1970
1st floor