单项选择题

__ planes in flight between airports, air traffic controllers rely on radar.

A．Tracked
B．To track
C．Being tracked
D．The tracking of
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单项选择题
Only later ___ what a terrible thing had happened.
A．they have realized
B．did they realize
C．they realized
D．they realize
单项选择题
He has won the first place, __ is clear from the expressions on his face.
A．that
B．as
C．what
D．when
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