疑问句的定语从句变成陈述句来看.
Is this factory big?变成陈述句是This factory is big.
同理,这句话变成陈述句是This factory is ____ you visited a few years ago.
此时,定语从句变成跟在is这个be动词后了,没有先行词,所以要补充一个先行词the one,先行词在从句中做宾语.you visited the one(factory),因此关系代词that,which都可以省略.
如题目是,Is this the factory...那边变成陈述句就是This is the factory _____you visited a few years ago.此时,定语从句的先行词就是the factory,同理,做从句的宾语,可以用that,which或者省略.