回答关于美国的一些问题。

1. Is the U. S. near China?

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2. What does the flag of the U. S. look like?

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3. What colour is the flag of the U. S.?

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4. What do they speak in the U. S.?

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简述心力衰竭治疗中“减轻心脏负荷”的具体措施。