单项选择题

Passage Two
Questions 30 to 32 are based on the passage you have just heard.

A．To forbid grazing on marginal lands.
B．To plant more trees on marginal lands.
C．To establish good ground cover.
D．To graze animals rotationally.
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单项选择题
The achievements for HIV vaccine has been given discount because ______.
A．it causes a number of questions to answer
B．it can’t effectively protect people from HIV
C．it can’t be recognized by the people
D．it is proved ineffective in treating HIV
填空题
From the first paragraph, we can learn the problem with America is that ______.
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