There is a big dog -----to a fence outside the house.为什么不选C?

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【译文】
There is a big dog _____to a fence outside the house.
有一只大狗被拴在屋外的栅栏上.
tied, 过去分词后置作定语,表示与前名词的被动关系,“被拴在...”
tie to 把……系在 ; 停泊
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