单项选择题

This is an illness that can result in total blindness _________ left untreated.

A．after
B．if
C．since
D．unless
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单项选择题
The whole embarrassing _________ in the bar was a great disappointment to them and set a bad example of English hospitality.
A．incident
B．accident
C．thing
D．matter
单项选择题
The National Geographic Society headquarters, _________ in Washington, D. C., is well- known for its magazine.
A．situated
B．placed
C．stood
D．positioned
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