单项选择题

By the sixth century AD, when few Europeans could read, the Chinese ______ paper.

A．invented
B．had invented
C．have invented
D．had been invented
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单项选择题
It isn't quite ______ that he will be present at meeting.
A．sure
B．right
C．exact
D．certain
单项选择题
It is highly desirable that a new president ______ for this university.
A．is appointed
B．will be appointed
C．be appointed
D．has been appointed
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