I will never forget the days ----we spent studying English.A.which B.whenC.as D.in that
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B，时间状语从句，修饰前面的the days.
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答案的确为B,但when引导的不是时间状语从句而是定语从句。when可以引导定语从句（有时可以用that代替；有时可以省略。）例如：Do you know the date when Lincoln was born? 你知道林肯的出生日期吗？I glanced at my watch and realized that it was time (when) I left. 我瞅了一眼手表，才发觉是我该离开的时间了。There was a time when all scientists were willing to share their results. 曾经有一段时间，所有的科学家都愿意与人共享他们的成果。
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B
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B
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B，时间状语从句，修饰前面的the days
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B
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B
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Ｂ
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Ｂ is right
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Ｂ状语从句．
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B.when =on which ----we spent studying English on the days .spend (in) doing sth. on sth.