单项选择题

Not until 1959 ______ this compound.

A．that chemists succeeded in obtaining
B．did chemists succeed to obtain
C．chemists succeeded in obtaining
D．did chemists succeed in obtaining
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单项选择题
As they searched the room, they looked for ______ to the crime.
A．labels
B．scenes
C．clues
D．tags
单项选择题
Robots are used ______ a lot of work in place of man.
A．to do
B．to doing
C．doing
D．to be done
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