单项选择题

The basic causes are unknown, although certain conditions that may lead to cancers have been ______ .

A．identified
B．guaranteed
C．notified
D．conveyed
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单项选择题
Bill is an example of a severely disabled person who has become ______ at many survival skills.
A．proficient
B．persistent
C．consistent
D．sufficient
单项选择题
Because of a ______ in her hearing, the teacher gave her a seat in the front row.
A．error
B．mistake
C．blunder
D．defect
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