单项选择题

I spent half an hour ______ this difficult math problem.

A．having worked on
B．to work on
C．on work
D．working on
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单项选择题
We use plastics ______ wood and metal now.
A．to take place
B．to take of
C．take the place of
D．in place of
单项选择题
Three people, ______, were injured in the accident.
A．including a child
B．include a child
C．included a child
D．includes a child
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