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
You can go swimming（）condition that you do not go far from the river bank.

A．on
B．in
C．under
D．with
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