is this the factory（ where ）he worked ten years ago?

第一句先行词指地点,关系副词在从句中作地点状语,所以用where.
第二句先行词虽然指地点,关系代词在从句中作visit的宾语,用that.
不一定先行词指地点、时间或reason时,就用关系副词when、where、why了,一定要看关系代词在从句中作的成分,如果关系代词在从句中作的是主语或宾语,仍然用which或that.比较下列各组句子：
I like to take a vacation in the mountains,which is quiet and beautiful.
I like to take a vacation in the mountains,where there are a lot of plants.
Never will I forget the day which we spent together.
Never will I forget the day when we worked together.
That is the reason that my son gave me for not going to college.
That is the reason why my son decided not to go to college.
希望我能帮助你解疑释惑.

看从句。
第一个句子的从句还原是： He worked [ in the factory ] ten years ago. 缺少的是一个介词短语，做状语。缺少状语要用关系副词 where, when 等引导。
第二个句子的从句还原是： You visited ( the factory ) the other day. 缺少的是一个名词，做宾语。缺少宾语要用关系代词 which...

两个句子都是对的。
1 先行词是THE FACTORY，引导词作状语（ WORK 不及物动词），故用WHERE
2 先行词也是THE FATORY， 引导词作VISIT（及物动词） 的宾语，故用THAT
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请点击本页面中的“选为满意回答”按钮，谢谢！第一个从句是否是完整的句子对, 是完整的句子...