单项选择题

The barrier of language keeps many scientists in prolonged ignorance of work of immediate importance to them ______at present in many countries.

A．doing
B．to be done
C．having done
D．being done
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单项选择题
In the Unites States, it is required that all children______ school, public or private, for 12 years
A．must attend
B．need attend
C．should attend
D．ought to attend
单项选择题
I spent the next two years investigating the very questions______ by him.
A．posed
B．held
C．put
D．set
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