我在网上查了一下，你是在定语从句中漏掉了主语she，原题是这样的：

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

答案是D。

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。