单项选择题

We still know little about outer ______ though we have made much effort in research.

A．heaven
B．universe
C．space
D．sky
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单项选择题
Good lighting in factories leads to greater comfort, higher ______ and productivity, fewer mistakes and accidents.
A．achievement
B．proficiency
C．craft
D．efficiency
单项选择题
This was the last public ______ the president made before his term of office ended.
A．appearance
B．engagement
C．gathering
D．conference
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