Of the 9,500 drivers since the beginning of the year, 23% were found in road knowledge.

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A. tested; lacked
B. testing; lacked
(答案→)C. tested; lacking
D. to test; lacking
解析：考查非谓语动词：句意：在今年年初以来被检测的9500个司机中，23%被发现缺乏公路知识。第一空做后置定语，修饰drivers，因为drivers和test是被动关系，用过去分词做定语，第二空是主语补足语，主语是the drivers和lack是主动关系，用现在分词做补足语，选C。