Not until the early years of the 19th century _________．
Not until the early years of the 19th century _________．
A.man did know heat is what
B.man know what is heat
C.did man know what heat is
D.didn’t man know heat
A.man did know heat is what
B.man know what is heat
C.did man know what heat is
D.didn’t man know heat
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