1.Is this the factory----you visited last year?则用which/that 2.Is this the factory----you worked l

第一句定语从句visited 是及物动词缺宾语,当然选关系代词which/ that 了
第二句定语从句worked 是不及物动词,可见从句缺的是状语,用where (=in which) 【 worked in the factory】为什么不及物动词，就是缺状语呢？不及物动词，由于本身意思完整，不需接什么宾语。关键是你把factory和worked 连起来看，worked the factory 行吗？ 肯定得说 worked in the factory 才行。 而而如果你只用which, 关系代词which 只代先行词the factory, 这就无异于说worked the factory, 而 where 等于in which ，就相当于说worked in the factory 了， 明白？where= in the factory 不是地点状语吗？ 慢慢看， 你会明白的。 ：）