问题描述：
1-------What is behind that tree -------There mu
1-------What is behind that tree
-------There must be a dog (sitting) behind it .
2.The telephone (inventing) by Alexander Bell is useful for people.
问:为什么1句sit要用现在分词?为什么2句invent要用ing形式?
1-------What is behind that tree
-------There must be a dog (sitting) behind it .
2.The telephone (inventing) by Alexander Bell is useful for people.
问:为什么1句sit要用现在分词?为什么2句invent要用ing形式?
问题解答：
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