单项选择题

There is supposed to be a safety ______ which makes it impossible for trains to collide.

A．appliance
B．accessory
C．machine
D．mechanism
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单项选择题
I don't doubt ______ he will come on time.
A．whether
B．which
C．what
D．that
单项选择题
Rumors are everywhere, spreading fear, damaging reputations, and turning calm situations into ______ ones.
A．turbulent
B．tragic
C．vulnerable
D．suspicious
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