In his letter Jack said that the days ________ he spent at college was the happiest time ________ he had ever had all his life．A.when; whenB.when; thatC.that; whichD.that; that
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D解析该题包含两个定语从句。第一个从句中spent需要宾语，应由that/which引导；第二个从句中had也需要宾语，但先行词有最高级修饰，只能由that引导。