单项选择题

The basic causes are unknown, although certain conditions that may lead to cancer have been（）

A. identified
B. guaranteed
C. notified
D. conveyed

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单项选择题
All individuals are required to（）to the laws made by their governments.

A. obey
B. conform
C. concede
D. observe
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
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