单项选择题

______ some mammals came to live in the sea is not known.

A．Which only
B．Since
C．Although
D．How
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单项选择题
One should always remember that appearances can be ______ .
A．wrong
B．mistaken
C．incorrect
D．false
单项选择题
When Mr. Green retired, his son ______ the business.
A．took up
B．took on
C．took over
D．took out
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