关于一个长难句里被动的用法

在沪江关注出国留学的沪友yhyheroUC遇到了一个关于新托福的疑惑,并悬赏500沪元,已有2人提出了自己的看法

知识点疑惑描述:

是一个托福的句子Records show merchant caravans and trading posts set up by the Sumerians in the surrounding mountains and deserts of Persia and Arabia, where they traded grain for raw materials, such as timber and stones, as well as for metals and gems.里面,posts set up by the 为啥不用posts were set up by呢,这个被动语态不是更好么

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set的过去分词还是set,其实是作为posts的后置定语来用的,所以没有助动词
—— suzuka1234
这个句子前面是一个简单句,set up by...是后置定语,那records是主语,show是谓语动词,merchant caravans and trading posts 是宾语。where引导的非限制定语从句中说明deserts of Persia and Arabia,where 是修饰地点的。这个句子也可以用被动,句子Records show merchant caravans and trading posts which were set up by the Sumerians in the surrounding mountains and deserts of Persia and Arabia, where they traded grain for raw materials, such as timber and stones, as well as for metals and gems.希望我的解说能对您有帮助(十年前想考托福,却因为现实放弃,祝你好运。)
—— lotusLI88