I can never forget the days we worked together and the days we spent happily together.A.

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(答案→)A. when; which
B. which; when
C. for which; that
D. on which; when
解析：考查定语从句。本题第一个定语从句的先行词是the days，后面的定语从句句子结构很完整，故使用关系副词when；第二空的先行词是the days，后面的定语从句中谓语动词spend后面缺少宾语，故使用关系代词that/which；句意：我永远不能忘记我们一起工作的那些日子，和我们在一起开心度过的日子。故A正确。