给你编个号：
1、Sep 18 1937 is the day ___ we will never forget.
本句定语从句,that在从句作宾语,即forget的宾语,做宾语的关系代词通常省略.
2、Isthis factory ___ we visited last year?
加上the one,是作定语从句的先行词 其实本该有that或which跟在the one后面的,道理和第一题一样,做宾语的关系代词省掉了.还原为陈述句看看：This factory is the one (that/which) we visited last year.
3、This is the factory ___ we visited last year.
这一句和我上面打的陈述句比比看,这里the factory自身就是先行词,所以可填which或that作从句里的宾语,或者省略不填.
4、Mr.Herpin is one of the foreign experts who ＿＿ in China
A works B is working C are working D has been woking
选C,这种情况,先行词应认为是就近的the foreign experts,所以谓语用复数,其余选项均是单数.
5、Mr.Smith is the only one of those foreiners who ＿＿ inChina.
A work B is working C are working D have been working
选B.此时因为one被only所修饰,故此句的先行词要认为是the only one,自然从句里的谓语要用单数,只有B是单数,其余均是复数.