Question

1 The hotel _____during our holidays stands by the seaside.

Answer 1

1.we stayed at 为正确选项.

定语从句的先行词是the hotel,关系代词在从句里做宾语（做介词at的宾语）,故省去了.当然,也可以填which/that we stayed at.in that we stayed错在介词一旦前置,就不能用that；

2.when是对的.

定语从句选哪个关系代词或副词,取决于它在从句里充当的成分,而选that或which的前提条件是,定语从句要缺少主语,宾语或者表语.但楼主的句子,定语从句we worked together并不缺少上文所提的三种成分,故不能选that或which.

先行词是the days,表示时间,且从句缺少时间状语,那么关系副词非when莫属了.