All reference books ， he didn't find the key to the question.
A. being searched
B. searched
C. to be searched
D. were searched
网友回答
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。