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？

谢谢~~~

Answer 1

during which time等于during when,用以引导非限制性定语从句，属于介词+when/where引导定语从句的范畴。
如果作为先行词的时间名词在主句、从句中都是作介词宾语，充当时间状语，但介词前后发生了变化，那么，定语从句的引导词就要使用“介词+ when /which time”（介词的使用要取决于从句中的搭配需要）。
如果作为先行词的时间名词在主句中作主语、宾语（作介词宾语时，主句从句中的介词是同一个介词）、表语等，加上适当的介词后在定语从句中时间状语，那么引导词就要使用“介词+which”（介词的使用要取决于从句中的搭配需要）。
①They stayed with me for two weeks, ____they drank all the beer I had.
A. for which B. which time C. during which time D. during which

②The book was written in 1946, ___ the education system has witnessed great changes.
A. when B. since which
C. since then D. since when

有类似用法的还有“介词+where":
① He stood at the window, ____ he could see what was happening.
A. from where B. from which C.which D. at which
② His head soon appeared out of one of the second storey window, from where he could see nothing but trees．