问题补充：
The students busily when Miss Brown went to get a book she in the office.
A. had written, left
B. were writing, has left
C. had written, had left
D. were writing, had left
答案：
A. had written, left
B. were writing, has left
C. had written, had left
(答案→)D. were writing, had left
解析：“把书忘在办公室”发生在“去取书”这一过去的动作之前，因此“忘了书”这一动作发生在过去的过去，用过去完成时。句中when表示的是时间的一点，表示在“同学们正忙于……”这一背景下，when所引导的动作发生。因此前一句应用过去进行时。
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