单项选择题

Not until the early years of the 19th century ______what heat is.

A．man did know
B．man knew
C．didn't man know
D．did man know
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单项选择题
Tom ought not to ______me your secret, but he meant no harm.
A．have told
B．tell
C．be telling
D．having told
单项选择题
______ she is young, she knows quite a lot.
A．When
B．However
C．Although
D．Unless
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