Mary will never forget the day___ she found her son who had gone missing three years before.

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Mary will never forget the day ___ she found her son who had gone missing three years before.
A.when B.which C.where D.that
day后面为一个定语从句修饰它,先行词是day
(when) she found her son who had gone missing two years before
= (on which) she found her son who had gone missing two years before
= (on the day) she found her son who had gone missing two years before
她永远也不会忘记她找到失踪了三年的儿子的那一天.
很明显,该定语从句缺少时间状语.
即所谓的关系副词when = 介词(on) + 关系代词which = 介词(on) + 先行词(the day).
而关系代词that/which只能等于先行词the day,无法直接放入定语从句中做成分；
where在定语从句中充当地点状语,而不是时间状语.
答案是【A】
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