单项选择题

When the main system fails, the backup system

A．take off.
B．take out.
C．take in.
D．take over.
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单项选择题
Vigor or weakness results ______ from energy or carelessness.
A．consequently
B．conversely
C．respectively
D．previously
单项选择题
An American painter is commissioned to paint the Empress’
A．cartoon.
B．portrait.
C．diagram.
D．sketch.
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