问题描述：
My grandfather suffered a lot during World War Two___ broke out in the 1939.
y grandfather suffered a lot during World War Two___ broke out in the 1939.
这里为什么填which,而不填 when ?
y grandfather suffered a lot during World War Two___ broke out in the 1939.
这里为什么填which,而不填 when ?
问题解答：
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