单项选择题

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

A．man did know
B．man know
C．didn't man know
D．did man know
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单项选择题
The weather in China is different from______.
A．in America
B．one in America
C．America
D．that in America
单项选择题
You will be late______ you leave immediately.
A．unless
B．until
C．if
D．or
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