This is the hotel ---last month .A .Where they stayed at ; B

This is the hotel ___last month .A .Where they stayed at ; B.Where they stayed 为什么答案是B
为什么不加at呢?不是stay at 是固定词组吗?Can you lend me the book___the other day?A.about which you talked B.which you talked C.about that you talked.D that you talked 为什么这个题选A 是不是跟他搭配的about要加在which的前面?