单项选择题

Training astronauts ______ not an easy thing.

A．are
B．is
C．were
D．be
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单项选择题
The heart is an important organ of circulation ______ function is to pump blood to all parts of the body.
A．which
B．whose
C．of it
D．its
单项选择题
I am rather concerned ______ her, for I haven' t heard from her since last winter.
A．of
B．at
C．with
D．about
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