单项选择题

To some extent, the principle of secrecy has been ______

A．maintained
B．held
C．observed
D．implemented
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单项选择题
Scientists say that many factors ______ great changes in the weather.
A．brought about
B．brought on
C．brought up
D．brought forward
单项选择题
In many areas, rapid development is the result of ______ in the research laboratory.
A．work to do
B．work done
C．doing work
D．work doing
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