All reference books he didn't find the key to the question.A. being searchedB. searched

问题补充：

All reference books ， he didn find the key to the question.

A. being searched

B. searched

C. to be searched

D. were searched

答案：

A. being searched

(答案→)B. searched

C. to be searched

D. were searched

解析：B考察非谓语动词。句义：所有的参考书都查遍了，他还是没有找到问题的答案。该句中的，reference books与search之间构成被动关系，而且表示动作已经完成，使用过去分词做状语。故B正确。