B
A
1.This is the school (that) we visited three years ago.
visit是及物动词,visit some place,所以这个句子是that引导的宾语从句,that可以省略
2.This is the factory where we worked a year ago.
=This is the factory in which we worked a year ago.
work是不及物动词,work in some place,所以这句是where引导的地点状语从句