单项选择题

The defense system will ______ if no proper identifying signal is given.

A．maintain
B．compact
C．condense
D．activate
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单项选择题
There is a way ______ he can be saved.
A．whereas
B．whereby
C．while
D．thereafter
单项选择题
At last Iris left her house and got to the railway station, only ______ the train pulling out of the station.
A．saw
B．having seen
C．to see
D．to have seen
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