that he had fallen behind he tried to work harder.A. Having realizedB. RealizedC. Realize

问题补充：

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

A. Having realized

B. Realized

C. Realize

D. To realize

答案：

(答案→)A. Having realized

B. Realized

C. Realize

D. To realize

解析：考查非谓语动词。句意：由于意识到他已经落后了，他尽力努力工作起来。现在分词做状语表示主语正在进行的伴随性动作，如果动作发生在谓语动作之前，则要用完成时态的现在分词。结合语境可知意识到落后这个动作发生在尽力工作之前，故选用完成时态的现在分词。选A。