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
When it was my turn to introduce myself, I was so nervous that my mind went ______.
A．blank
B．faint
C．vain
D．dim'
单项选择题
They claimed that they did not arrange the demonstration. It was absolutely ______.
A．spontaneous
B．simultaneous
C．synthetic
D．substantial
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