I had been looking for the book for two days I found it at last.AuntilBwhenCwhil

问题补充：

I had been looking for the book for two days I found it at last.

Auntil

Bwhen

Cwhile

D. before

答案：

A. until

B. when

C. while

(答案→)D. before

解析：考查连词：A. until直到，B. when当…时候，C. while当…时候，虽然，然而，D. before在…前面，…才…，因为前面是过去完成式，所以是发生在过去的动作前的动作，句意：在我最后找到它之前，我已经找了这本书两天了。或是：我找了这本书两天才找到。选D。