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

问题补充：

that he had fallen behind, he tried to work harder.

A. Realize

B. Realized

C. To realize

D. Having realized

答案：

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。