When was the Willis Tower built?
1970
1939
1927
1939
1927
1968
1964
1970s
1939
1968
1927
1963
1932
1970
1960
1926
1964
1931
1968
1939
1963
1970
1927
1960s
1924
1961
1960
1964
1932
1971
1962
1965
1970s
1968
1967
1926
1931
1973
1963
1970
1923
1961
1939
1960
1964
1927
1971
1962
1924
1965
1968
1967
1932
1969
1973
1926
1963
1970
1960s
1961
1931
1960
1964
1923
1971
1972
1962
1965
1970s
1939
1968
1967
1927
1924
1969
1973
1963
1932
1921
1970
1961
1960
1926
1964
1971
1972
1962
1965
1931
1968
1967
1923
1969
1973
1939
1963
1966
1976
1970
1927
1960s
1924
1961
1957
1960
1964
1932
1971
1972
1962
1965
1970s
1968
1967
1926
1969
1931
1973
1959
1963
1966
1976
1970
1923
1974
1961
1980
1958
1957
1939
1960
1964
1927
1971
1922
1972
1962
1924
1965
1968
1967
1932
1921
1970s
1969
1973
1982
1926
1959
1963
1966
1976
1970
1974
1961
1931
1980
1958
1957
1960
1964
1955
1923
1971
1972
1962
1965
1978
1939
1968
1967
1975
1977
1927
1922
1924
1969
1973
1982
1959
1963
1966
1932
1976
1970
1974
1970s
1961
1980
1958
1957
1960
1926
1964
1955
1971
1956
1972
1962
1965
1986