单项选择题

A country must always be ________ guard against spies and invaders.

A．in
B．to
C．with
D．on
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单项选择题
Living things are ________ cells, and cells do not grow to more than twice the size they were at first.
A．made from
B．made up of
C．made up from
D．made of
单项选择题
No sooner had I closed the door than somebody started knocking ________ it.
A．on
B．with
C．to
D．for
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