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2.—Bird flu(禽流感)is still very serious in some countries.

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2.—Bird flu(禽流感)is still very serious in some countries.
—Yes.Something must __________(do) to stop it from spreading.
done .但我不明白为什么要用be done,这是什么时态?为什么不能用be doing
2.—Bird flu(禽流感)is still very serious in some countries.
这个是被动语态.直译就是:“什么事情一定要被做”.被动语态肯定要用过去分词.do的过去分词是done.