that he had fallen behind, he tried to work harder. A. RealizeB. RealizedC. To realizeD.

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A. Realize
B. Realized
C. To realize
(答案→)D. Having realized
解析：非谓语做状语时不定式表目的，过去分词表被动，现在分词表主动。本题中主语he与realize为主谓关系所以用现在分词做状语。“意识到落后了，他比以前更努力”选D。