单项选择题

What is the passage mainly about

A．How biochemical changes occur in the human brain.
B．Why biochemical changes Occur in the human brain.
C．How intellectual activities influence brain-cell health.
D．Why people should keep active not only physically but also mentally.

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单项选择题

A．goodB．easierC．more difficultD．efficient

A．good
B．easier
C．more difficult
D．efficient

单项选择题

[Al be said [B] was said [C] is being said [D] had been said

A．[Al be said
B．was said
C．is being said
D．had been said