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D. "把書忘在辦公室"發(fā)生在"去取書"這一過去的動(dòng)作之前，因此"忘了書"這一動(dòng)作發(fā)生在過去的過去，用過去完成時(shí)。句中when表示的是時(shí)間的一點(diǎn)，表示在"同學(xué)們正忙于……"這一背景下，when所引導(dǎo)的動(dòng)作發(fā)生。因此前一句應(yīng)用過去進(jìn)行時(shí)。

　　注意：

　　had no … when　還沒等…… 就……

　　had no sooner… than　剛…… 就……

　　He had no sooner bought the car than he sold it.