The second world war ____ millions of people were killed ended in 1945.
A.when B.in which
那两个选项不是一样的。
那句话能够加个逗号之后理解：The Second World War ____ millions of people were killed， ended in 1945.
也能够拆成两句话来理解：
The Second World War ended in 1945.
Millions of people were killed in the Second World War.
in which与where差不多，那边不是指时间，而是指地点。跟when一点关系也没有，代进去也欠亨，对吧？
后面millions of people were killed 是对前面The second world war做的限制语的，选B，暗示那场数百万人被杀的二战
The second world war _B___ millions of people were killed ended in 1945.
A.when B.in which
when是时间状语从句引导词，前面说的是世界大战中（事务），即指物。所以可与跟 in which
或that即millions of people were killed in the second world war ended in 1945
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