单项选择题

A．15 minutes.
B．30 minutes.
C．45 minutes.
D．60 minutes.
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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
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
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