已收到你的求助,答案选【C】
这里考察两个语法点
【1】定语从句
【2】It's ···that···的强调句
首先 将疑问句改为陈述句来看
It could be in the room where we had a talk last night that you left your keys.
然后划分出强调句型
【It could be】 in the room where we had a talk last night 【that】 you left your keys.
再然后分析定语从句
the room where we had a talk last night
这里从句完整,we【主语】 had 【谓语】a talk【宾语】 last night【时间状语】
缺少【地点状语】,故采用where或者in which都是可以的,两者等价（关系副词=关系代词+介词）
希望帮助到了你!