Question

they stayed with me for two weeks,____they did all the housework

they stayed with me for two weeks,____they did all the housework
A、at which B、during when C、during which D、during that time

答案是C
我想知道为什么不能用at？
为什么要用which？不能用when吗？they did all the housework句子结构不是完整了吗，结构完整不是应该用when吗？

请详细解答谢谢~~~

Answer 1

这句是个非限制性定语从句，先行词是two weeks, 而后面的定语从句缺个时间状语，应该是这样：They did all the housework during two weeks. 同时during提前，指物的情况下只能用which，不能用at，at表示一般表示具体的时间点，不搭配。在这里也可以直接用when。when= during which.

Answer 2

因为这里是想要表达“他们在那里呆了两周，在这两周当中他们完成了所有的作业”
重点是“两周”， “在这两周当中”
which 就是指two weeks
而during which 就是during the two weeks 在这两周当中

而at which一般是表示某个时间点的，而不是某一段时间

这句子中间有个逗号，属于非限制性定语从句，一般会用介词+which这种形式来
如果没有了逗号，可以是they stayed with me for two weeks when they did all the housework
因为when可以相应地变成during which/ in which/ on which/ at which