--- Did you return the book to me? Why can’t I find it anywhere?
--- ________. You were writing a letter at your desk at that time.A.I will give it to youB.I did give it to youC.I had given it to youD.I do give it to you
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B解析根据下文时间状语在那个时候，可知前面叙述的是过去发生的事情，故用过去时态，选B。