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 students ______ busily when Miss Brown went to get a book she ______ in the office.
A．had written, left
B．were writing, has left
C．had written, had left
D．were writing, had left
单项选择题
Tom failed the exam but his sister ______.
A．got in
B．got over
C．got out
D．got through
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