I'll remember the time (______ we spent together in the country).
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主语 谓语 宾语 定语从句
解析：主句：I'll remember the time.【宾语the time被后置定语(从句)限定】
从句：______ we spent together in the country.
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主语 谓语 状语
从成分可以看出：从句缺宾语,应该是spent (the time→that/which),代词提前引导从句.
I'll remember the time ______ we spent together in the country.
A.when B.at which C.that D.during which
正确答案：C (还可选which)
峰子回答,希望对朋友有帮助,什么情况会使用到during which？谢谢~during之后的which指代the time，所以during which=during the time，若用在句中相当于时间状语，这里需要的是宾语，不需要加介词。若还是不明白，请继续追问，谢谢！