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
Do you know the boy ______ under the big tree7
A．lay
B．lain
C．laying
D．lying
单项选择题
The weather turned out to be very good, ______ was more than we could expect.
A．what
B．which
C．that
D．if
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