Not until the early years of the 19th ce
问题:
Not until the early years of the 19th century ________ what heat was.
A.man did know
B.did man knew
C.didn't man know
D.did man know
Not until the early years of the 19th century ________ what heat was.
A.man did know
B.did man knew
C.didn't man know
D.did man know