单项选择题

From paragraph 1, we know that some 40 genes involving in cancer are （）

A．harmful to the human body
B．necessary to the human body
C．the elements that form cancer
D．useless to the human body

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单项选择题
The title that best expresses the main idea of this passage is （）

A．Knowledge Is Power
B．How to Retain and Use Knowledge
C．Physical and Mental Efforts
D．The Trained Mind Is What One Needs
单项选择题
21（）

A．down
B．up
C．away
D．off
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